Patent Publication Number: US-2020286767-A1

Title: Electrostatic chuck and processing apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2019-039861, filed on Mar. 5, 2019, and No. 2020-013599, filed on Jan. 30, 2020; the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments of the invention described herein relate to an electrostatic chuck and a processing apparatus. 
     BACKGROUND 
     A ceramic electrostatic chuck is fabricated by sandwiching an electrode between ceramic dielectric substrates made of e.g. alumina, followed by firing. Electric power for electrostatic suction is applied to the incorporated electrode. Thus, the electrostatic chuck sucks a substrate such as a silicon wafer by electrostatic force. In such an electrostatic chuck, an inert gas such as helium (He) is passed between the front surface of the ceramic dielectric substrate and the back surface of the suction target substrate to control the temperature of the suction target substrate. 
     For instance, the temperature increase of the substrate may be associated with processing in a device for processing a substrate such as a CVD (chemical vapor deposition) device, sputtering device, ion implantation device, and etching device. In the electrostatic chuck used in such devices, an inert gas such as He is passed between the ceramic dielectric substrate and the suction target substrate to bring the substrate into contact with the inert gas. Thus, the temperature increase of the substrate is suppressed. 
     In the electrostatic chuck for controlling the substrate temperature with an inert gas such as He, a hole (gas feed channel) for feeding an inert gas such as He is provided in the ceramic dielectric substrate and a base plate for supporting the ceramic dielectric substrate. The ceramic dielectric substrate is provided with a through hole communicating with the gas feed channel of the base plate. Thus, the inert gas fed from the gas feed channel of the base plate is guided through the through hole of the ceramic dielectric substrate to the back surface of the substrate. 
     Here, when the substrate is processed in the device, electric discharge (arc discharge) may occur from the plasma in the device toward the metallic base plate. The gas feed channel of the base plate and the through hole of the ceramic dielectric substrate may be likely to constitute a path of discharge. Thus, there is known a technique in which a porous part is provided in the gas feed channel of the base plate and the through hole of the ceramic dielectric substrate to improve resistance (such as breakdown voltage) to arc discharge. For instance, an electrostatic chuck is proposed, in which an insulative property in the gas feed channel is improved by providing a ceramic sintered porous body in the gas feed channel and using the structure and film pore of the ceramic sintered porous body as a gas flow path. An electrostatic chuck is proposed, in which a discharge prevention member made of a ceramic porous body for a process gas flow path is provided in a gas diffusion gap in order to prevent the discharge. An electrostatic chuck is proposed, in which the arc discharge is reduced by providing a dielectric insert as a porous dielectric such as alumina. A technique is proposed, in which a plurality of fine pores communicating with gas supply pores are provided by laser processing. 
     In such an electrostatic chuck, the arc discharge is desired to be further reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic sectional view illustrating an electrostatic chuck according to the first embodiment; 
         FIGS. 2A to 2D  are schematic views illustrating the electrostatic cuck according to the embodiment; 
         FIGS. 3A and 3B  are schematic views illustrating a porous part of the electrostatic cuck according to the embodiment; 
         FIG. 4  is a schematic plan view illustrating the porous part of the electrostatic chuck according to the embodiment; 
         FIG. 5  is a schematic plan view illustrating the porous part of the electrostatic chuck according to the embodiment; 
         FIGS. 6A and 6B  are schematic plan views illustrating the porous part of the electrostatic chuck according to the embodiment; 
         FIGS. 7A and 7B  are schematic views illustrating a first porous part  90  according to the other embodiment; 
         FIG. 8  is a schematic sectional view illustrating the electrostatic chuck according to the embodiment; 
         FIGS. 9A and 9B  are schematic sectional views illustrating the electrostatic chuck according to the embodiment; 
         FIG. 10  is a schematic sectional view illustrating the porous part of the electrostatic chuck according to the embodiment; 
         FIG. 11  is a schematic sectional view illustrating a porous part according to the other embodiment; 
         FIGS. 12A and 12B  are schematic sectional views illustrating the porous part according to the other embodiment; 
         FIGS. 13A to 13D  are schematic sectional views illustrating the porous parts according to the other embodiment; 
         FIGS. 14A to 14C  are schematic sectional views illustrating the porous parts according to the other embodiment; 
         FIGS. 15A and 15B  are schematic sectional views illustrating the porous parts according to the other embodiment; 
         FIG. 16  is a schematic sectional view illustrating an electrostatic chuck according to the other embodiment; 
         FIGS. 17A and 17B  are enlarged views of the region C shown in  FIG. 16 ; 
         FIG. 18  is a schematic sectional view illustrating a plurality of holes according to the other embodiment; 
         FIGS. 19A and 19B  are schematic sectional views illustrating shapes of opening portion; 
         FIG. 20  is a schematic sectional view illustrating the electrostatic chuck according to the other embodiment; 
         FIG. 21  is a schematic sectional view illustrating the electrostatic chuck according to the other embodiment; 
         FIG. 22  is a schematic sectional view illustrating the electrostatic chuck according to the other embodiment; 
         FIG. 23  is an enlarged view of the region E shown in  FIG. 22 ; 
         FIG. 24  is an enlarged view of the region E shown in  FIG. 22  of the other embodiment; and 
         FIG. 25  is a schematic view illustrating a processing apparatus according to the embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The first invention is an electrostatic chuck including: a ceramic dielectric substrate; a base plate; and a first porous part. The ceramic dielectric substrate includes a first major surface and a second major surface. A suction target is placed on the first major surface. The second major surface is opposite to the first major surface. The base plate supports the ceramic dielectric substrate and includes a gas feed channel. The first porous part is provided in the ceramic dielectric substrate and is opposite to the gas feed channel. The first porous part includes a first porous region, and a first dense region denser than the first porous region. The first porous region includes a plurality of first sparse portions, and a first dense portion. The plurality of first sparse portions includes a plurality of pores. The first dense portion has a density higher than a density of the first sparse portions. A dimension in a second direction is smaller than a dimension of the first dense region in the second direction. Each of the plurality of first sparse portions extends in a first direction from the base plate toward the ceramic dielectric substrate. The first dense portion is positioned between the plurality of first sparse portions. The first sparse portions include a first wall part provided between the plurality of pores. A minimum value of a dimension of the first wall part is smaller than a minimum value of the dimension of the first dense portion in the second direction substantially orthogonal to the first direction. At least a part of an edge of an opening on the ceramic dielectric substrate side of the gas feed channel is configured to be a curve. 
     According to the electrostatic chuck, the first porous part includes the first sparse portions extending in the first direction and the first dense portion. This can improve a mechanical strength (rigidity) of the first porous part while ensuring an arc discharge resistance and a gas flow rate. At least a part of an edge of an opening of the gas feed channel is configured to be a curve. This can suppress electric field concentration, and further can reduce the arc discharge. 
     The second invention is an electrostatic chuck including: a ceramic dielectric substrate; a base plate; and a second porous part. The ceramic dielectric substrate includes a first major surface and a second major surface. A suction target is placed on the first major surface. The second major surface is opposite to the first major surface. The base plate supports the ceramic dielectric substrate and includes a gas feed channel. The second porous part is provided on the base plate and is opposite to the gas feed channel. The second porous part includes a second porous region, and a second dense region denser than the second porous region. The second porous region includes a plurality of second sparse portions, and a second dense portion. The plurality of second sparse portions includes a plurality of pores. The second dense portion has a density higher than a density of the second sparse portions. A dimension in a second direction is smaller than a dimension of the second dense region in the second direction. Each of the plurality of second sparse portions extends in a first direction from the base plate toward the ceramic dielectric substrate. The second dense portion is positioned between the plurality of second sparse portions. The second sparse portions include a second wall part provided between the plurality of pores. A minimum value of a dimension of the second wall part is smaller than a minimum value of the dimension of the second dense portion in the second direction substantially orthogonal to the first direction. At least a part of an edge of an opening on the ceramic dielectric substrate side of the gas feed channel is configured to be a curve. 
     According to the electrostatic chuck, the second porous part includes the second sparse portions extending in the first direction and the second dense portion. This can improve a mechanical strength (rigidity) of the second porous part while ensuring an arc discharge resistance and a gas flow rate. At least a part of an edge of an opening of the gas feed channel is configured to be a curve. This can suppress electric field concentration, and further can reduce the arc discharge. 
     The third invention is the electrostatic chuck of the first or second invention, wherein a dimension of the plurality of pores provided in each of the plurality of first sparse portions is smaller than the dimension of the first dense portion in the second direction, and/or a dimension of the plurality of pores provided in each of the plurality of second sparse portions is smaller than the dimension of the second dense portion in the second direction. 
     According to the electrostatic chuck, the arc discharge can be reduced. 
     The fourth invention is the electrostatic chuck of one of the first to third inventions, wherein an aspect ratio of the plurality of pores provided in each of the plurality of first sparse portions, and/or an aspect ratio of the plurality of pores provided in each of the plurality of second sparse portions is 30 or more. 
     According to the electrostatic chuck, the arc discharge resistance can be further improved. 
     The fifth invention is the electrostatic chuck of one of the first to fourth inventions, wherein in the second direction, a dimension of the plurality of pores provided in each of the plurality of first sparse portions and/or a dimension of the plurality of pores provided in each of the plurality of second sparse portions is not less than 1 micrometer and not more than 20 micrometers. 
     According to the electrostatic chuck, pores having a pore dimension of 1 to 20 micrometers and extending in one direction can be arranged. This can realize a high resistance to the arc discharge. 
     The sixth invention is the electrostatic chuck of one of the first to fifth inventions, wherein as viewed along the first direction, a plurality of pores provided in the first sparse portions include a first pore positioned at a center of the first sparse portions, among the plurality of pores, a number of pores adjacent to the first pore and surrounding the first pore is 6, and/or as viewed along the first direction, a plurality of pores provided in the second sparse portions include a second pore positioned at a center of the second sparse portions, among the plurality of pores, a number of pores adjacent to the second pore and surrounding the second pore is 6. 
     According to the electrostatic chuck, in a plan view, a plurality of pores can be arranged with a high isotropy and a high density. This can improve the rigidity of the first porous part while ensuring the resistance to arc discharge and the gas flow rate. 
     The seventh invention is the electrostatic chuck of one of the first to sixth inventions, wherein the ceramic dielectric substrate includes a first hole part positioned between the first major surface and the first porous part, at least one of the ceramic dielectric substrate or the first porous part includes a second hole part positioned between the first hole part and the first porous part, and in a second direction substantially orthogonal to the first direction, a dimension of the second hole part is smaller than a dimension of the first porous part, and larger than a dimension of the first hole part. 
     According to the electrostatic chuck, the first porous part is provided at a position opposing to the gas feed channel. This can improve the resistance to arc discharge while ensuring the flow rate of the gas flowing in the first hole part. The second hole part having a predetermined dimension is provided. This can introduce most gas introduced into the first porous part with a large dimension into the first hole part with a small dimension through the second hole part. That is, the arc discharge can be reduced and the gas flow can be smoothed. 
     The eighth invention is the electrostatic chuck of one of the first to seventh inventions, wherein the first porous region further includes a first dense part, the first dense part having a density higher than a density of the first sparse portions and a dimension in the second direction being larger than a dimension of a dense portion in the second direction, and as projected on a plane perpendicular to the first direction, the first dense part overlaps the first hole part. 
     According to the electrostatic chuck, the dense part and the first hole part are configured to overlap, thus a generated current tries to flow around the dense part. For that reason, a distance (conduction path) where current flows can be made long, and thus an electron is not easy to be accelerated, and further the occurrence of arc discharge can be suppressed. 
     The ninth invention is the electrostatic chuck of the eighth invention, wherein as projected on the plane perpendicular to the first direction, the plurality of first sparse portions are provided around the first dense part. 
     According to the electrostatic chuck, the occurrence of arc discharge can be suppressed more effectively while ensuring the gas flow. 
     The tenth invention is the electrostatic chuck of the eighth or ninth invention, wherein a length along the first direction of the first dense part is smaller than a length along the first direction of the first porous part. 
     According to the electrostatic chuck, the occurrence of arc discharge can be suppressed more effectively while ensuring the gas flow. 
     The eleventh invention is the electrostatic chuck of one of the eighth to tenth inventions, wherein in the first direction, the sparse portions are provided between the first dense part and the base plate. 
     According to the electrostatic chuck, the gas flow can be smoothed while suppressing the occurrence of arc discharge. The twelfth invention is the electrostatic chuck of the eighth or ninth invention, a length along the first direction of the first dense part is substantially equal to a length along the first direction of the first porous part. 
     According to the electrostatic chuck, a length along the first direction of the dense part is made substantially as long as a length along the first direction of the first porous part, thus the occurrence of arc discharge can be suppressed more effectively. 
     The thirteenth invention is the electrostatic chuck of one of the first to twelfth inventions, wherein each of the plurality of first sparse portions extends in a direction inclined by a predetermined angle to the first direction. 
     According to the electrostatic chuck, each of the plurality of first sparse portions extends in the direction inclined by the predetermined angle to the first direction, thus the electron is considered to be not easy to be accelerated when the current flows in the pore provided in the first sparse portions. Thus, the occurrence of arc discharge can be effectively suppressed. 
     The fourteenth invention is the electrostatic chuck of the thirteenth invention, wherein the plurality of pores provided in each of the plurality of sparse portions are configured to flow a gas introduced from the gas feed channel. 
     According to the electrostatic chuck, the gas introduced from the gas feed channel can be introduced to the first major surface side of the ceramic dielectric substrate. 
     The fifteenth invention is the electrostatic chuck of the thirteenth or fourteenth invention, wherein an angle between the first direction and an extending direction of the first sparse portions is not less than 5° and not more than 30°. 
     According to the electrostatic chuck, the occurrence of arc discharge is easy to be suppressed. 
     The sixteenth invention is a processing apparatus: including one of the electrostatic chucks described above; and a supplier configured to supply a gas into a gas feed channel provided in the electrostatic chuck. 
     According to the processing apparatus, the arc discharge can be reduced. 
     Various embodiments are described below with reference to the accompanying drawings. 
     In the drawings, similar components are marked with like reference numerals, and a detailed description is omitted as appropriately. 
     In the drawings, a direction from a base plate  50  toward a ceramic dielectric substrate  11  is taken as a Z-direction (corresponding to one example of a first direction), one of directions substantially orthogonal to the Z-direction is taken as a Y-direction (corresponding to one example of a second direction), and a direction substantially perpendicular to the Z-direction and the Y direction is taken as an X-direction (corresponding to one example of the second direction). 
     (Electrostatic Chuck) 
       FIG. 1  is a schematic sectional view illustrating an electrostatic chuck according to the embodiment. 
     As shown in  FIG. 1 , an electrostatic chuck  110  according to the embodiment includes the ceramic dielectric substrate  11 , the base plate  50 , and a porous part  90 . In this example, the electrostatic chuck  110  further includes a porous part  70 . 
     The ceramic dielectric substrate  11  is e.g. a flat plate-like base material made of sintered ceramic. For instance, the ceramic dielectric substrate  11  includes aluminum oxide (Al 2 O 3 ). For instance, the ceramic dielectric substrate  11  is formed of high-purity aluminum oxide. The concentration of aluminum oxide in the ceramic dielectric substrate  11  is e.g. not less than 99 atomic percent (atomic %) and not more than 100 atomic %. Use of high-purity aluminum oxide can improve the plasma resistance of the ceramic dielectric substrate  11 . The ceramic dielectric substrate  11  has a first major surface  11   a  on which a target W (suction target) is placed, and a second major surface  11   b  on the opposite side from the first major surface  11   a . The target W is e.g. a semiconductor substrate such as a silicon wafer. 
     The ceramic dielectric substrate  11  is provided with an electrode  12 . The electrode  12  is provided between the first major surface  11   a  and the second major surface  11   b  of the ceramic dielectric substrate  11 . The electrode  12  is formed so as to be inserted in the ceramic dielectric substrate  11 . A power supply  210  is electrically connected to the electrode  12  via a connection part  20  and a wiring  211 . By application of a suction-holding voltage to the electrode  12  from the power supply  210 , charge is generated on the first major surface  11   a  side of the electrode  12  and the target W can be suction-held by electrostatic force. 
     The electrode  12  is shaped like a thin film along the first major surface  11   a  and the second major surface  11   b  of the ceramic dielectric substrate  11 . The electrode  12  is a suction electrode for suction-holding the target W. The electrode  12  may be of the unipolar type or the bipolar type. The electrode  12  illustrated in  FIG. 1  is of the bipolar type, with electrodes  12  of two polarities provided on the same plane. 
     The electrode  12  is provided with a connection part  20  extending to the second major surface  11   b  side of the ceramic dielectric substrate  11 . The connection part  20  is e.g. a via (solid type) or via hole (hollow type) in electrical continuity with the electrode  12 . The connection part  20  may be a metal terminal connected by a suitable method such as brazing. The base plate  50  is a member for supporting the ceramic dielectric substrate  11 . The ceramic dielectric substrate  11  is fixed on the base plate  50  with a bonding part  60  illustrated in  FIG. 2A . The bonding part  60  can be e.g. a cured silicone adhesive. 
     The base plate  50  is e.g. metallic. The base plate  50  is e.g. divided into an upper part  50   a  and a lower part  50   b  made of aluminum. A communication channel  55  is provided between the upper part  50   a  and the lower part  50   b . One end side of the communication channel  55  is connected to an input channel  51 . The other end side of the communication channel  55  is connected to an output channel  52 . The base plate  50  may include a sprayed part (not shown) on an end on the second major surface  11   b  side. The sprayed part is formed by thermal spraying, for instance. The sprayed part may constitute an end surface (upper surface  50 U) on the second major surface  11   b  side of the base plate  50 . The sprayed part is provided as necessary, and may be omitted. 
     The base plate  50  also serves to adjust the temperature of the electrostatic chuck  110 . For instance, in the case of cooling the electrostatic chuck  110 , a cooling medium is caused to flow in from the input channel  51 , to pass through the communication channel  55 , and to flow out from the output channel  52 . This can absorb heat from the base plate  50  by the cooling medium to cool the ceramic dielectric substrate  11  attached onto the base plate  50 . On the other hand, in the case of keeping warm the electrostatic chuck  110 , a heat-retaining medium can be put into the communication channel  55 . Alternatively, a heating element can be incorporated in the ceramic dielectric substrate  11  or the base plate  50 . Thus, the temperature of the base plate  50  and the ceramic dielectric substrate  11  is adjusted. This can adjust the temperature of the target W suction-held by the electrostatic chuck  110 . 
     Dots  13  are provided as necessary on the first major surface  11   a  side of the ceramic dielectric substrate  11 . A groove  14  is provided between the dots  13 . That is, the first major surface  11   a  is a protrusion-depression surface and includes a depression and a protrusion. The protrusion of the first major surface  11   a  corresponds to the dot  13 . The depression of the first major surface  11   a  corresponds to the groove  14 . For instance, the groove  14  can extend continuously in the X-Y plane. Thereby, a gas such as He or the like can be distributed over the whole first major surface  11   a . A space is formed between the back surface of the target W placed on the electrostatic chuck  110  and the first major surface  11   a  including the groove  14 . 
     The ceramic dielectric substrate  11  includes a through hole  15  connected to the groove  14 . The through hole  15  is provided from the second major surface  11   b  to the first major surface  11   a . That is, the through hole  15  extends in the Z-direction from the second major surface  11   b  to the first major surface  11   a  and penetrates through the ceramic dielectric substrate  11 . The through hole  15  includes, for instance, a hole part  15   a , a hole part  15   b , a hole part  15   c , a hole part  15   d  (described later in detail). 
     The height of the dot  13  (the depth of the groove  14 ), the area ratio between the dots  13  and the grooves  14 , the shapes thereof and the like can be appropriately selected to control the temperature of the target W and particles attached to the target W in a desirable state. 
     The base plate  50  is provided with a gas feed channel  53 . For instance, the gas feed channel  53  is provided so as to penetrate through the base plate  50 . The gas feed channel  53  may not penetrate through the base plate  50 , but may branch halfway from another gas feed channel  53  and extend to the ceramic dielectric substrate  11  side. The gas feed channel  53  may be provided at a plurality of locations in the base plate  50 . 
     The gas feed channel  53  communicates with the through hole  15 . That is, the gas (such as helium (He)) flowing into the gas feed channel  53  passes through the gas feed channel  53 , and then flows into the through hole  15 . 
     The gas flowing into the through hole  15  passes through the through hole  15 , and then flows into the space provided between the target W and the first major surface  11   a  including the groove  14 . This can directly cool the target W with the gas. 
     The first porous part  90  can be provided at a position e.g. between the base plate  50  and the first major surface  11   a  of the ceramic dielectric substrate  11  in the Z-direction. The porous part  90  can be provided at a position opposed to the gas feed channel  53 . For instance, the porous part  90  is provided in the through hole  15  of the ceramic dielectric substrate  11 . For instance, the first porous part  90  is inserted into the through hole  15 . 
       FIGS. 2A and 2B  are schematic views illustrating the electrostatic chuck according to the embodiment.  FIG. 2A  illustrates the neighborhood of the porous part  90  and the porous part  70 .  FIG. 2A  corresponds to an enlarged view of region A shown in  FIG. 1 .  FIG. 2B  is a plan view illustrating the porous part  90 . 
       FIGS. 2C and 2D  are schematic sectional views for illustrating the hole part  15   c  and the hole part  15   d  according to the other embodiment. 
     In order to avoid complexity, the dots  13  (see e.g.  FIG. 1 ) are omitted in  FIGS. 2A, 2C, and 2D . 
     In this example, the through hole  15  includes the hole part  15   a  and the hole part  15   b  (the first hole part). One end of the hole part  15   a  is positioned on the second major surface  11   b  of the ceramic dielectric substrate  11 . 
     The ceramic dielectric substrate  11  can include the hole part  15   b  positioned between the first major surface  11   a  and the porous part  90  in the Z-direction. The hole part  15   b  communicates with the hole part  15   a  and extends to the first major surface  11   a  of the ceramic dielectric substrate  11 . That is, one end of the hole part  15   b  is positioned on the first major surface  11   a  (a bottom surface  14   a  of the groove  14 ). The hole part  15   b  is positioned between the first major surface  11   a  of the ceramic dielectric substrate  11  and the porous part  90 . The hole part  15   b  is a link hole for linking the first porous part  90  and the groove  14 . The diameter (length along the X-direction) of the hole part  15   b  is smaller than the diameter (length along the X-direction) of the hole part  15   a . Providing a hole part  15   b  having a small diameter can improve the design flexibility of the space formed between the ceramic dielectric substrate  11  and the target W (e.g. the first major surface  11   a  including the groove  14 ). For instance, as shown in  FIG. 2A , the width (length along the X-direction) of the groove  14  can be made shorter than the width (length along the X-direction) of the porous part  90 . This can suppress discharge in e.g. the space formed between the ceramic dielectric substrate  11  and the target W. 
     The diameter of the hole part  15   b  is e.g. not less than 0.05 millimeters (mm) and not more than 0.5 mm. The diameter of the hole part  15   a  is e.g. not less than 1 mm and nor more than 5 mm. The hole part  15   b  may communicate indirectly with the hole part  15   a . That is, a hole part  15   c  (the second hole part) may be provided to connect the hole part  15   a  and the hole part  15   b . As illustrated in  FIG. 2A , the hole part  15   c  can be provided in the ceramic dielectric substrate  11 . Alternatively, as illustrated in  FIG. 2C , the hole part  15   c  can be provided in the porous part  90 . Alternatively, as illustrated in  FIG. 2D , the hole part  15   c  can be provided in the ceramic dielectric substrate  11  and the porous part  90 . That is, at least one of the ceramic dielectric substrate  11  and the porous part  90  can include a hole part  15   c  located between the hole part  15   b  and the porous part  90 . In this case, when the hole part  15   c  is provided in the ceramic dielectric substrate  11 , the strength around the hole part  15   c  can be increased, and the occurrence of e.g. chipping can be suppressed around the hole part  15   c . This can suppress the occurrence of arc discharge more effectively. When the hole part  15   c  is provided in the porous part  90 , the hole part  15   c  is easily aligned with the porous part  90 . This further facilitates the compatibility between reduction of arc discharge and smoothing of the flow of gas. Each of the hole part  15   a , the hole part  15   b , and the hole part  15   c  is shaped like e.g. a circular cylinder extending in the Z-direction. 
     In this case, in the X-direction or the Y-direction, the dimension of the hole part  15   c  can be made smaller than the dimension of the porous part  90  and larger than the dimension of the hole part  15   b . In the electrostatic chuck  110  according to the embodiment, the porous part  90  is provided at a position opposed to the gas feed channel  53 . This can improve resistance to arc discharge while ensuring the flow rate of the gas flowing in the hole part  15   b . The dimension in the X-direction or the Y-direction of the hole part  15   c  is made larger than the dimension of the hole part  15   b . Thus, most of the gas fed into the porous part  90  having a large dimension can be fed through the hole part  15   c  into the hole part  15   b  having a small dimension. That is, reduction of arc discharge can be made compatible with smoothing of the flow of gas. 
     As described above, the ceramic dielectric substrate  11  includes at least one groove  14  opened to the first major surface  11   a  and communicating with the through hole  15 . The dimension of the hole part  15   c  in the Z-direction can be smaller than the dimension of the groove  14  in the Z-direction. This can reduce the time taken by the gas to pass through the hole part  15   c . That is, the occurrence of arc discharge can be suppressed more effectively while smoothing the flow of gas. In the X-direction and the Y-direction, the dimension of the hole part  15   c  can be larger than the dimension of the groove  14 . Thus, the gas can be made easy to flow into the groove  14 . This makes it possible to effectively cool the target W by the gas. 
     It is favorable to make an arithmetic average surface roughness Ra of a surface  15   c   1  (ceiling surface) on the first major surface  11   a  side smaller than an arithmetic average surface roughness Ra of the bottom surface  14   a  (a surface on the second major surface  11   b  side) of the groove  14 . In this way, since there is no large unevenness on the surface  15   c   1  of the hole part  15   c , the occurrence of arc discharge can be suppressed more effectively. 
     It is favorable to make an arithmetic average surface roughness Ra of the surface  14   a  on the second major surface  11   b  side on the groove  14  smaller than an arithmetic average surface roughness of the second major surface  11   b . In this way, since there is no large unevenness on the surface  14   a  of the grove  14 , the occurrence of arc discharge can be suppressed more effectively. 
     The hole part  15   c  (third hole part) provided between the hole part  15   b  and the hole part  15   c  can be further included. In the X-direction or the Y-direction, the dimension of the hole part  15   d  can be made larger than the hole part  15   b , and smaller than the hole part  15   c . Providing the hole part  15   d  can make the flow of gas smooth. 
     As described above, a bonding part  60  can be provided between the ceramic dielectric substrate  11  and the base plate  50 . In the Z-direction, the dimension of the hole part  15   c  can be made smaller than the dimension of the bonding part  60 . This can improve the bonding strength between the ceramic dielectric substrate  11  and the base plate  50 . The dimension of the hole part  15   c  in the Z-direction is made smaller than the dimension of the bonding part  60 . Thus, the occurrence of arc discharge can be suppressed more effectively while smoothing the flow of gas. 
     In this example, the porous part  90  is provided in the hole part  15   a . Thus, the upper surface  90 U of the porous part  90  is not exposed to the first major surface  11   a . That is, the upper surface  90 U of the porous part  90  is located between the first major surface  11   a  and the second major surface  11   b . On the other hand, the lower surface  90 L of the porous part  90  is exposed to the second major surface  11   b.    
     Next, the porous part  90  will be described. The porous part  90  includes a plurality of sparse portions  94  and a plurality of dense portions  95  described later.  FIG. 2  illustrates a case of providing the porous part  90  in the ceramic substrate  11 , however as described later, the porous part  90  may be provided in the base plate  50  (for instance,  FIG. 12B  etc.). 
     The porous part  90  includes a porous region  91  one example of a first porous region, a second porous region) including a plurality of pores  96 , and a dense region  93  (one example of a first dense region, a second dense region) denser than the porous region  91 . The porous region  91  is configured to allow the gas flow. The gas flows inside each of the plurality of pores  96 . The dense region  93  is a region having pores  96  less than the porous region  91  or a region having substantially no pore  96 . The porosity (percent: %) of the dense region  93  is lower than the porosity (%) of the porous region  91 . The density (gram/cubic centimeter: g/cm 3 ) of the dense region  93  is higher than the density (g/cm 3 ) of the porous region  91 . Since the dense region  93  is dense in comparison with the porous region  91 , for instance, the rigidity (mechanical strength) of the dense region  93  is higher than the rigidity of the porous region  91 . 
     The porosity of the dense region  93  is e.g. a proportion that the volume of space (pore  96 ) included in the dense region  93  occupies in the total volume of the dense region  93 . The porosity of the porous region  91  is e.g. a proportion that the volume of space (pore  96 ) included in the porous region  91  occupies in the total volume of the porous region  91 . For instance, the porosity of the porous region  91  is not less than 5% and not more than 40%, favorably not less than 10% and not more than 30%, and the porosity of the dense region  93  is not less than 0% and not more than 5%. 
     The porous part  90  is shaped like a column (e.g. circular column). The porous region  91  is shaped like a column (e.g. circular column). The dense region  93  is in contact with the porous region  91 , or is continuous with the porous region  91 . As shown in  FIG. 2B , as projected on a plane (XY-plane) perpendicular to the Z-direction, the dense region  93  surrounds the outer periphery of the porous region  91 . The dense region  93  is shaped like a cylinder (e.g. circular cylinder) surrounding the side surface  91   s  of the porous region  91 . In other words, the porous region  91  is provided so as to penetrate through the dense region  93  in the Z-direction. The gas flowing from the gas feed channel  53  into the through hole  15  passes through the plurality of pores  96  provided in the porous region  91  and is supplied to the groove  14 . 
     The porous part  90  includes the porous region  91  as described above. This can improve resistance to arc discharge while ensuring the flow rate of the gas flowing in the through hole  15 . The porous part  90  includes the dense region  93 . This can improve the rigidity (mechanical strength) of the porous part  90 . 
     When the porous part  90  is provided in the ceramic dielectric substrate  11 , for instance, the first porous part  90  may be integrated with the ceramic dielectric substrate  11 . The state in which two members are integrated refers to the state in which the two members are chemically coupled by e.g. sintering. No material (e.g. adhesive) for fixing one member to the other is provided between the two members. That is, in this example, no other member such as adhesive is provided between the porous part  90  and the ceramic dielectric substrate  11 . Thus, the porous part  90  and the ceramic dielectric substrate  11  are integrated with each other. 
     Thus, when the first porous part  90  is fixed to the ceramic dielectric substrate  11  by integration with the ceramic dielectric substrate  11 , the strength of the electrostatic chuck  110  can be improved compared with the case of fixing the porous part  90  to the ceramic dielectric substrate  11  with e.g. adhesive. For instance, there is no degradation of the electrostatic chuck due to e.g. corrosion or erosion of adhesive. 
     When the porous part  90  and the ceramic dielectric substrate  11  are integrated with each other, the side surface of the outer periphery of the porous part  90  may be subjected to a force from the ceramic dielectric substrate  11 . On the other hand, when the porous part  90  is provided with a plurality of pores to ensure the flow rate of gas, the mechanical strength of the porous part  90  is decreased. Thus, when the porous part is integrated with the ceramic dielectric substrate  11 , the porous part  90  may be broken by the force applied from the ceramic dielectric substrate to the porous part  90 . 
     On the contrary, the porous part  90  includes the dense region  93 . This can improve the rigidity (mechanical strength) of the porous part  90 , and the porous part  90  can be integrated with the ceramic dielectric substrate  11 . 
     In the embodiment, the porous part  90  does not necessarily need to be integrated with the ceramic dielectric substrate  11 . For instance, as shown in  FIG. 11 , the porous part  90  may be attached to the ceramic dielectric substrate with adhesive. 
     The dense region  93  is positioned between the inner wall  15   w  of the ceramic dielectric substrate  11  forming the through hole  15 , and the porous region  91 . That is, the porous region  91  is provided inside the porous part  90 . The dense region  93  is provided outside the porous part  90 . This can improve the rigidity against the force applied from the ceramic dielectric substrate  11  to the porous part  90 . Thus, the porous part  90  and the ceramic dielectric substrate  11  can be easily integrated with each other. For instance, a bonding member  61  (see  FIG. 11 ) may be provided between the porous part  90  and the ceramic dielectric substrate  11 . In this case, the dense region  93  can suppress that the bonding member  61  is exposed to the gas passing in the porous part  90 . This can suppress degradation of the bonding member  61 . The porous region  91  is provided inside the porous part  90 . This can suppress that the through hole  15  of the ceramic dielectric substrate  11  is occluded with the dense region  93 . Thus, the flow rate of gas can be ensured. 
     The thickness of the dense region  93  (length L 0  between the side surface  91   s  of the porous region  91  and the side surface  93   s  of the dense region  93 ) is e.g. not less than 100 μm and not more than 1000 μm. 
     The material of the porous part  90  is an insulative ceramic. The porous part  90  (each of the porous region  91 , and the dense region  93 ) includes at least one of aluminum oxide (Al 2 O 3 ), titanium oxide (TiO 2 ), and yttrium oxide (Y 2 O 3 ). This can achieve high breakdown voltage and high rigidity of the porous part  90 . 
     For instance, the porous part  90  is composed primarily of one of aluminum oxide, titanium oxide, and yttrium oxide. 
     In this case, the purity of aluminum oxide of the ceramic dielectric substrate  11  can be made higher than the purity of aluminum oxide of the porous part  90 . This can ensure the performance of the electrostatic chuck  110  such as plasma resistance, and ensure the mechanical strength of the porous part  90 . As an example, a trace amount of additive is contained in the porous part  90 . This facilitates sintering the porous part  90 , and can control the pores and ensure the mechanical strength. 
     In the specification, the ceramic purity of e.g. aluminum oxide of the ceramic dielectric substrate  11  can be measured by e.g. fluorescent X-ray analysis or ICP-AES method (inductively coupled plasma-atomic emission spectrometry). 
     For instance, the material of the porous region  91 , and the material of the dense region  93  are the same. However, the material of the porous region  91 , and the material of the dense region  93  may be different. The composition of the material of the porous region  91 , and the composition of the material of the dense region  93  may be different. 
       FIGS. 3A and 3B  are schematic views illustrating the first porous part of the electrostatic chuck according to the embodiment. 
       FIG. 3A  is a plan view of the first porous part  90  as viewed along the Z-direction.  FIG. 3B  is a sectional view taken along Z-Y plane of the porous part  90 . 
     As shown in  FIG. 3A  and  FIG. 3B , in the porous part  90 , the porous region  91  includes a plurality of sparse portions  94  (one example of a first sparse portion and a second sparse portion) and a dense portion  95  (one example of a first dense portion and a second dense portion). The porous part  90  may include a plurality of dense portions  95 . Each of the plurality of sparse portions  94  includes a plurality of pores  96 . The dense portion  95  is denser than the sparse portion  94 . That is, the dense portion  95  is a portion including fewer pores than the sparse portion  94 , or a portion including substantially no pores. The dimension of the dense portion  95  in the X-direction or the Y-direction is smaller than the dimension of the dense region  93  in the X-direction or the Y-direction. The porosity of the dense portion  95  is lower than the porosity of the sparse portion  94 . Thus, the density of the dense portion  95  is higher than the density of the sparse portion  94 . The porosity of the dense portion  95  may be equal to the porosity of the dense region  93 . The dense portion  95  is denser than the sparse portion  94 . Thus, the rigidity of the dense portion  95  is higher than the rigidity of the sparse portion  94 . 
     The porosity of one sparse portion  94  is e.g. the proportion that the volume of the space (pores  96 ) included in that sparse portion  94  occupies in the total volume of that sparse portion  94 . The porosity of the dense portion  95  is e.g. the proportion that the volume of the space (pores  96 ) included in the dense portion  95  occupies in the total volume of the dense portion  95 . For instance, the porosity of the sparse portion  94  is not less than 20% and not more than 60%, and preferably not less than 30% and not more than 50%. The porosity of the dense portion  95  is not less than 0% and not more than 5%. 
     Each of the plurality of sparse portions  94  extends in the Z-direction. For instance, each of the plurality of sparse portions  94  is shaped like a column (e.g. circular column or polygonal column) and provided so as to penetrate through the porous region  91  in the Z-direction. The dense portion  95  is located between the plurality of sparse portions  94 . The dense portion  95  is shaped like a wall partitioning between the sparse portions  94  neighboring each other. As shown in  FIG. 3A , as projected on a plane (XY-plane) perpendicular to the Z-direction, the dense portion  95  is provided so as to surround the outer periphery of each of the plurality of sparse portions  94 . The dense portion  95  is continuous with the dense region  93  at the outer periphery of the porous region  91 . 
     The number of sparse portions  94  provided in the porous region  91  is e.g. not less than 50 and not more than 1000. As shown in  FIG. 3A , as projected on a plane (XY-plane) perpendicular to the Z-direction, the plurality of sparse portions  94  have a size generally equal to each other. For instance, as projected on a plane (XY-plane) perpendicular to the Z-direction, the plurality of sparse portions  94  are dispersed isotropically and uniformly in the porous region  91 . For instance, the distance between the neighboring sparse portions  94  (i.e. the thickness of the dense portion  95 ) is generally constant. 
     For instance, as projected on a plane (XY-plane) perpendicular to the Z-direction, the distance L 11  between the side surface  93   s  of the dense region  93  and the sparse portion  94  of the plurality of sparse portions  94  nearest to the side surface  93   s  is not less than 100 μm and not more than 1000 μm. 
     Thus, by providing a plurality of sparse portions  94 , and a dense portion  95  denser than the sparse portion  94  in the porous region  91 , the rigidity of the porous part  90  can be improved while ensuring resistance to arc discharge and the flow rate of the gas flowing in the through hole  15  compared with the case where a plurality of pores are dispersed randomly in three dimensions in the porous region. 
     For instance, the increase of the porosity of the porous region  91  results in increasing the flow rate of gas, but decreasing arc discharge resistance and rigidity. In contrast, by providing the dense portion  95  in the porous region  91 , which has a smaller dimension in the X-direction or the Y-direction than a dimension of the dense region  93  in the X-direction or the Y-direction, the decrease of arc discharge resistance and rigidity can be suppressed even when the porosity is increased. 
     For instance, as projected on a plane (XY-plane) perpendicular to the Z-direction, suppose a minimum circle, ellipse, or polygon containing all the plurality of sparse portions  94 . The inside of the circle, ellipse, or polygon can be regarded as the porous region  91 . The outside of the circle, ellipse, or polygon can be regarded as the dense region  93 . 
     As described above, the porous part  90  can include a plurality of sparse portions  94  and a dense portion  95 . The plurality of sparse portions  94  include a plurality of pores  96  including a first pore  96  and a second pore  96 . The dense portion  95  has a higher density than the sparse portion  94 . 
     Each of the plurality of sparse portions  94  extends in the Z-direction. The dense portion  95  is positioned between the plurality of sparse portions  94 . The sparse portion  94  includes a wall part  97  (one example of a first wall part, a second wall part) provided between the plurality of pores  96  (between the first pore  96  and the second pore  96 ). In the X-direction or the Y-direction, the minimum dimension of the wall part  97  can be made smaller than the minimum dimension of the dense portion  95 . Thus, the first porous part  90  is provided with the sparse portions  94  and the dense portion  95  extending in the Z-direction. This can improve the mechanical strength (rigidity) of the first porous part  90  while ensuring arc discharge resistance and gas flow rate. 
     As illustrated in  FIG. 5  described later, in the X-direction or the Y-direction, the dimension of the plurality of pores  96  provided in each of the plurality of sparse portions  94  can be made smaller than the dimension of the dense portion  95 . Thus, the dimension of the plurality of pores  96  can be made sufficiently small. This can further improve resistance to arc discharge. 
     The aspect ratio of the plurality of pores  96  provided in each of the plurality of sparse portions  94  can be set to not less than 30 and not more than 10000. This can further improve resistance to arc discharge. More preferably, the lower limit of the aspect ratio of the plurality of pores  96  is 100 or more, and the upper limit is 1600 or less. 
     In the X-direction or the Y-direction, the dimension of the plurality of pores  96  provided in each of the plurality of sparse portions  94  can be set to not less than 1 micrometer and not more than 20 micrometers. Thus, the pores  96  having a pore dimension of 1-20 micrometers and extending in one direction can be arranged. This can achieve high resistance to arc discharge. 
     As shown in  FIGS. 6A and 6B  described later, as projected on a plane (XY-plane) perpendicular to the Z-direction, a pore  96   a  is positioned in a central part of the sparse portion  94 . Among the plurality of pores  96 , the number of pores  96   b - 96   g  neighboring the pore  96   a  and surrounding the pore  96   a  can be set to 6. Thus, in a plan view, a plurality of pores  96  can be arranged with high isotropy and high density. This can improve the rigidity of the porous part  90  while ensuring arc discharge resistance and gas flow rate. 
       FIG. 4  is a schematic plan view illustrating the porous part  90  of the electrostatic chuck according to the embodiment. 
       FIG. 4  shows a part of the porous part  90  as viewed along the Z-direction, and corresponds to an enlarged view of  FIG. 3A . 
     As projected on a plane (XY-plane) perpendicular to the Z-direction, each of the plurality of sparse portions  94  is generally shaped like a hexagon (shaped like a generally regular hexagon). As projected on a plane (XY-plane) perpendicular to the Z-direction, the plurality of sparse portions  94  include a sparse portion  94   a  positioned at a center of the porous region  91  and six sparse portions  94  (second to seventh sparse portions  94   b - 94   g ) surrounding the sparse portion  94   a . The sparse portions  94   b - 94   g  neighbor the first sparse portion  94   a . The sparse portions  94   b - 94   g  are provided to be nearest to sparse portions  94   a  of the plurality of sparse portions  94 . 
     The sparse portion  94   b  and the sparse portion  94   c  are juxtaposed with the sparse portion  94   a  in the X-direction. That is, the sparse portion  94   a  is positioned between the sparse portion  94   b  and the sparse portion  94   c.    
     The length L 1  along the X-direction of the sparse portion  94   a  (the diameter of the sparse portion  94   a ) is longer than the length L 2  along the X-direction between the sparse portion  94   a  and the sparse portion  94   b , and longer than the length L 3  along the X-direction between the sparse portion  94   a  and the sparse portion  94   c.    
     Each of the length L 2  and the length L 3  corresponds to the thickness of the dense portion  95 . That is, the length L 2  is the length along the X-direction of the dense portion  95  between the sparse portion  94   a  and the sparse portion  94   b . The length L 3  is the length along the X-direction of the dense portion  95  between the sparse portion  94   a  and the sparse portion  94   c . The length L 2  and the length L 3  can be generally equal. For instance, the length L 2  can be not less than 0.5 times and not more than 2.0 times of the length L 3 . 
     The length L 1  can be generally equal to the length L 4  along the X-direction of the sparse portion  94   b  (the diameter of the sparse portion  94   b ). The length L 1  can be generally equal to the length L 5  along the X-direction of the sparse portion  94   c  (the diameter of the sparse portion  94   c ). For instance, each of the length L 4  and the length L 5  can be not less than 0.5 times and not more than 2.0 times of the length L 1 . 
     Thus, the sparse portion  94   a  neighbors and is surrounded with six sparse portions  94  of the plurality of sparse portions  94 . That is, as projected on a plane (XY-plane) perpendicular to the Z-direction, in the central part of the porous region  91 , the number of sparse portions  94  neighboring one sparse portion  94  is 6. Thus, in a plan view, a plurality of sparse portions  94  can be arranged with high isotropy and high density. This can improve the rigidity of the porous part  90  while ensuring arc discharge resistance and the flow rate of the gas flowing in the through hole  15 . This can also suppress variation in arc discharge resistance, variation in the flow rate of the gas flowing in the through hole  15 , and variation in the rigidity of the porous part  90 . 
     The diameter of the sparse portion  94  (e.g. length L 1 , L 4 , or L 5 ) is e.g. not less than 50 μm and not more than 500 μm. The thickness of the dense portion  95  (e.g. length L 2  or L 3 ) is e.g. not less than 10 μm and not more than 100 μm. The diameter of the sparse portion  94  is larger than the thickness of the dense portion  95 . As described previously, the thickness of the dense portion  95  is thinner than the thickness of the dense region  93 . 
       FIG. 5  is a schematic plan view illustrating the porous part of the electrostatic chuck according to the embodiment. 
       FIG. 5  shows a part of the porous part  90  as viewed along the Z-direction.  FIG. 5  is an enlarged view of the circumference of one sparse portion  94 . 
     As shown in  FIG. 5 , in this example, the sparse portion  94  includes a plurality of pores  96  and a wall part  97  provided between the plurality of pores  96 . 
     Each of the plurality of pores  96  extends in the Z-direction. Each of the plurality of pores  96  is shaped like a capillary extending in one direction (one-dimensional capillary structure), and penetrates through the sparse portion  94  in the Z-direction. The wall part  97  is shaped like a wall partitioning the pores  96  adjacent each other. As shown in  FIG. 5 , as projected on a plane (XY-plane) perpendicular to the Z-direction, the wall part  97  is provided so as to surround the outer periphery of each of the plurality of pores  96 . The wall part  97  is continuous with the dense portion  95  at the outer periphery of the sparse portion  94 . 
     The number of pores  96  provided in one sparse portion  94  is e.g. not less than 50 and not more than 1000. As shown in  FIG. 5 , as projected on a plane (XY-plane) perpendicular to the Z-direction, the plurality of pores  96  have a size generally equal to each other. For instance, as projected on a plane (XY-plane) perpendicular to the Z-direction, the plurality of pores  96  are dispersed isotropically and uniformly in the sparse portion  94 . For instance, the distance between the adjacent pores  96  (i.e. the thickness of the wall part  97 ) is generally constant. 
     Thus, the pores  96  extending in one direction are arranged in the sparse portion  94 . This can achieve high resistance to arc discharge with small variation compared with the case where a plurality of pores are dispersed randomly in three dimensions in the sparse portion. 
     Here, the “capillary structure” of the plurality of pores  96  is further described. 
     In recent years, for the purpose of high integration of semiconductor devices, the circuit line width is growing narrower, and the circuit pitch is growing finer. The electrostatic chuck is subjected to higher power. The temperature control of the target W is desired at higher level. Against this background, it is desired to ensure sufficient gas flow rate and to control the flow rate with high accuracy while reliably suppressing arc discharge in high-power environment. The electrostatic chuck  110  according to the embodiment includes a ceramic plug (porous part  90 ). The ceramic plug is conventionally provided to prevent arc discharge in the helium supply port (gas feed channel  53 ). In the embodiment, the pore diameter (the diameter of the pore  96 ) of the ceramic plug is decreased to the level of e.g. several to several ten μm (the details of the diameter of the pore  96  will be described later). The diameter decreased to this level may make it difficult to control the flow rate of gas. Thus, in the invention, for instance, the shape of the pore  96  is further devised so as to lie along the Z-direction. Specifically, in the conventional art, the flow rate is ensured using a relatively large pore, and its shape is made three-dimensionally complex to achieve prevention of arc discharge. In contrast, in the invention, the dimension of the pore  96  is made finer to the level of e.g. several to several ten μm to achieve prevention of arc discharge. Conversely, its shape is simplified to ensure flow rate. That is, the invention has been conceived based on the idea totally different from the conventional art. 
     The shape of the sparse portion  94  is not limited to the hexagon, but may be a circle (or ellipse) or other polygons. For instance, as projected on a plane (XY-plane) perpendicular to the Z-direction, suppose a minimum circle, ellipse, or polygon containing all the plurality of pores  96  arranged with a pitch of 10 μm or less. The inside of the circle, ellipse, or polygon can be regarded as the sparse portion  94 . The outside of the circle, ellipse, or polygon can be regarded as the dense portion  95 . 
       FIGS. 6A and 6B  are schematic plan views illustrating the porous part  90  of the electrostatic chuck according to the embodiment. 
       FIGS. 6A and 6B  show a part of the porous part  90  as viewed along the Z-direction, and are enlarged views showing the pores  96  in one sparse portion  94 . 
     As shown in  FIG. 6A , as projected on a plane (XY-plane) perpendicular to the Z-direction, the plurality of pores  96  include a pore  96   a  positioned in the central part of the sparse portion  94 , and six pores  96  (pores  96   b - 96   g ) surrounding the pore  96   a . The pores  96   b - 96   g  are adjacent to the pore  96   a . The pores  96   b - 96   g  are pores  96  of the plurality of pores  96  nearest to the pore  96   a.    
     The pore  96   b  and the pore  96   c  are aligned with the pore  96   a  in the X-direction. That is, the pore  96   a  is positioned between the pore  96   b  and the pore  96   c.    
     For instance, the length L 6  along the X-direction of the pore  96   a  (the diameter of the pore  96   a ) is longer than the length L 7  along the X-direction between the pore  96   a  and the pore  96   b , and longer than the length L 8  along the X-direction between the pore  96   a  and the pore  96   c.    
     Each of the length L 7  and the length L 8  corresponds to the thickness of the wall part  97 . That is, the length L 7  is the length along the X-direction of the wall part  97  between the pore  96   a  and the pore  96   b . The length L 8  is the length along the X-direction of the wall part  97  between the pore  96   a  and the pore  96   c . The length L 7  and the length L 8  can be generally equal. For instance, the length L 7  can be not less than 0.5 times and not more than 2.0 times of the length L 8 . 
     The length L 6  can be generally equal to the length L 9  along the X-direction of the pore  96   b  (the diameter of the pore  96   b ). The length L 6  can be generally equal to the length L 10  along the X-direction of the pore  96   c  (the diameter of the pore  96   c ). For instance, each of the length L 9  and the length L 10  can be not less than 0.5 times and not more than 2.0 times of the length L 6 . 
     For instance, when the diameter of the pore is small, arc discharge resistance and rigidity are improved. On the other hand, when the diameter of the pore is large, the flow rate of gas can be increased. The diameter of the pore  96  (e.g. length L 6 , L 9 , or L 10 ) is e.g. not less than 1 micrometer (μm) and not more than 20 μm. Thus, pores having a diameter of 1-20 micrometers and extending in one direction are arranged. This can achieve high resistance to arc discharge with small variation. More preferably, the diameter of the pore  96  is not less than 3 μm and not more than 10 μm. 
     Here, a method for measuring the diameter of the pore  96  is described. A scanning electron microscope (e.g. Hitachi High-Technologies, S-3000) is used to capture an image with a magnification of 1000 times or more. Commercially available image analysis software is used to calculate 100 circle-equivalent diameters for pores  96 . Their average value is used as the diameter of the pore  96 . 
     It is more preferable to suppress variation in the diameter of the plurality of pores  96 . By decreasing variation in the diameter, the flow rate of the flowing gas and the breakdown voltage can be controlled more precisely. The variation in the diameter of the plurality of pores  96  can be based on the cumulative distribution of the 100 circle-equivalent diameters obtained in the above calculation of the diameter of the pore  96 . Specifically, the concept of particle diameter D50 (median diameter) for the cumulative distribution 50 vol % and particle diameter D90 for the cumulative distribution 90 vol % are applied. These are generally used in granularity distribution measurement. The cumulative distribution graph for the pores  96  is produced in which the horizontal axis represents pore diameter (μm) and the vertical axis represents relative pore amount (%). This graph is used to determine the pore diameter for the cumulative distribution 50 vol % (corresponding to D50 diameter) and the pore diameter for the cumulative distribution 90 vol % (corresponding to D90 diameter). Preferably, the variation in the diameter of the plurality of pores  96  is suppressed so as to satisfy the relation D50:D90≤1:2. 
     The thickness of the wall part  97  (e.g. length L 7  or L 8 ) is e.g. not less than 1 μm and not more than 10 μm. The thickness of the wall part  97  is thinner than the thickness of the dense portion  95 . 
     Thus, the first pore  96   a  is adjacent and surrounded with six pores  96  of the plurality of pores  96 . That is, as projected on a plane (XY-plane) perpendicular to the Z-direction, in the central part of the sparse portion  94 , the number of pores  96  neighboring one pore  96  is 6. Thus, in plan view, a plurality of pores  96  can be arranged with high isotropy and high density. This can improve the rigidity of the porous part  90  while ensuring arc discharge resistance and the flow rate of the gas flowing in the through hole  15 . This can also suppress variation in arc discharge resistance, variation in the flow rate of the gas flowing in the through hole  15 , and variation in the rigidity of the first porous part  90 . 
       FIG. 6B  shows another example of the arrangement of the plurality of pores  96  in the sparse portion  94 . As shown in  FIG. 6B , in this example, the plurality of pores  96  are arranged concentrically about the first pore  96   a . Thus, as projected on a plane (XY-plane) perpendicular to the Z-direction, a plurality of pores can be arranged with high isotropy and high density. 
     Each of the lengths L 0 -L 10  can be measured by observation using a microscope such as a scanning electron microscope. 
     The evaluation of porosity in the specification is described. In the description, the evaluation of porosity in the porous part  90  is taken as an example. 
     An image like the plan view of  FIG. 3A  is captured. Image analysis is used to calculate the proportion R 1  of the plurality of sparse portions  94  occupied in the porous region  91 . The image is captured using a scanning electron microscope (e.g. Hitachi High-Technologies, S-3000). A BSE image is captured at an acceleration voltage of 15 kV and a magnification of 30 times. For instance, the image size is 1280×960 pixels, and the image gray scale assumes 256 levels. 
     The proportion R 1  of the plurality of sparse portions  94  occupied in the porous region  91  is calculated using image analysis software (e.g. Win-ROOF Ver. 6.5 (Mitani Corporation)). 
     Calculation of the proportion R 1  using Win-ROOF Ver. 6.5 can be performed as follows. 
     The evaluation range ROI 1  (see  FIG. 3A ) is set to the minimum circle (or ellipse) including all the sparse portions  94 . 
     Binarization by a single threshold (e.g. 0) is performed to calculate the area S 1  of the evaluation range ROI 1 . 
     Binarization by two thresholds (e.g. 0 and 136) is performed to calculate the total area S 2  of the plurality of sparse portions  94  in the evaluation range ROI 1 . At this time, filling in the sparse portions  94  and deletion of regions having a small area regarded as noise (threshold being 0.002 or less) are performed. The two thresholds are appropriately adjusted by the brightness and contrast of the image. 
     The proportion R 1  is calculated as the proportion of the area S 2  to the area S 1 . That is, the proportion R 1  is given by proportion R 1  (%)=(area S 2 )/(area S 1 )×100. 
     In the embodiment, the proportion R 1  of the plurality of sparse portions  94  occupied in the porous region  91  is e.g. not less than 40% and not more than 70%, and preferably not less than 50% and not more than 70%. The proportion R 1  is e.g. approximately 60%. 
     An image like the plan view of  FIG. 5  is captured. Image analysis is used to calculate the proportion R 2  of the plurality of pores  96  occupied in the sparse portion  94 . The proportion R 2  corresponds to e.g. the porosity of the sparse portion  94 . The image is captured using a scanning electron microscope (e.g. Hitachi High-Technologies, S-3000). A BSE image is captured at an acceleration voltage of 15 kV and a magnification of 600 times. For instance, the image size is 1280×960 pixels, and the image gray scale assumes 256 levels. 
     The proportion R 2  of the plurality of pores  96  occupied in the sparse portion  94  is calculated using image analysis software (e.g. Win-ROOF Ver. 6.5 (Mitani Corporation)). 
     Calculation of the proportion R 2  using Win-ROOF Ver. 6.5 can be performed as follows. 
     The evaluation range R 012  (see  FIG. 5 ) is set to a hexagon approximating the shape of the sparse portion  94 . The evaluation range R 012  includes all the pores  96  provided in one sparse portion  94 . 
     Binarization by a single threshold (e.g.  0 ) is performed to calculate the area S 3  of the evaluation range R 012 . 
     Binarization by two thresholds (e.g.  0  and  96 ) is performed to calculate the total area S 4  of the plurality of pores  96  in the evaluation range R 012 . At this time, filling in the pores  96  and deletion of regions having a small area regarded as noise (threshold being 1 or less) are performed. The two thresholds are appropriately adjusted by the brightness and contrast of the image. 
     The proportion R 2  is calculated as the proportion of the area S 4  to the area S 3 . That is, the proportion R 2  is given by proportion R 2  (%)=(area S 4 )/(area S 3 )×100. 
     In the embodiment, the proportion R 2  of the plurality of pores  96  occupied in the sparse portion  94  (the porosity of the sparse portion  94 ) is e.g. not less than 20% and not more than 60%, and preferably not less than 30% and not more than 50%. The proportion R 2  is e.g. approximately 40%. 
     The porosity of the porous region  91  corresponds to e.g. the product of the proportion R 1  of the plurality of sparse portions  94  occupied in the porous region  91  and the proportion R 2  of the plurality of pores  96  occupied in the sparse portion  94 . For instance, when the proportion R 1  is 60% and the proportion R 2  is 40%, the porosity of the porous region  91  can be calculated as approximately 24%. 
     Thus, the porous part  90  includes a porous region  91  having the porosity as described above. This can improve breakdown voltage while ensuring the flow rate of the gas flowing in the through hole  15 . 
     Likewise, the porosity of the ceramic dielectric substrate  11  and the porous part  70  can be calculated. Preferably, the magnification of the scanning electron microscope is appropriately selected within the range of several ten times to several thousand times depending on the observation target. 
       FIGS. 7A and 7B  are schematic views illustrating an alternative porous part  90  according to the embodiment. 
       FIG. 7A  is a plan view of the porous part  90  as viewed along the Z-direction.  FIG. 7B  corresponds to an enlarged view of part of  FIG. 7A . 
     As shown in  FIGS. 7A and 7B , in this example, the planar shape of the sparse portion  94  is circular. Thus, the planar shape of the sparse portion  94  does not need to be hexagonal. 
       FIG. 8  is a schematic sectional view illustrating the electrostatic chuck according to the embodiment. 
       FIG. 8  corresponds to an enlarged view of region B shown in  FIG. 2 . That is,  FIG. 8  shows the neighborhood of the interface F 1  between the porous part  90  (dense region  93 ) and the ceramic dielectric substrate  11 . In this example, the material of the porous part  90  and the ceramic dielectric substrate  11  is aluminum oxide. 
     As shown in  FIG. 8 , the porous part  90  includes a first region  90   p  located on the ceramic dielectric substrate  11  side in the X-direction or the Y-direction, and a second region  90   q  continuous with the first region  90   p  in the X-direction or the Y-direction. The first region  90   p  and the second region  90   q  are part of the dense region  93  of the first porous part  90 . 
     The first region  90   p  is located between the second region  90   q  and the ceramic dielectric substrate  11  in the X-direction or the Y-direction. The first region  90   p  is a region of approximately 40-60 μm in the X-direction or the Y-direction from the interface F 1 . That is, the width W 1  along the X-direction or the Y-direction of the first region  90   p  (the length of the first region  90   p  in the direction perpendicular to the interface F 1 ) is e.g. not less than 40 μm and not more than 60 μm. 
     The ceramic dielectric substrate  11  includes a first substrate region  11   p  located on the porous part  90  (first region  90   p ) side in the X-direction or the Y-direction, and a second substrate region  11   q  continuous with the first substrate region  11   p  in the X-direction or the Y-direction. The first region  90   p  and the first substrate region  11   p  are provided in contact with each other. The first substrate region  11   p  is located between the second substrate region  11   q  and the porous part  90  in the X-direction or the Y-direction. The first substrate region  11   p  is a region of approximately 40-60 μm in the X-direction or the Y-direction from the interface F 1 . That is, the width W 2  along the X-direction or the Y-direction of the first substrate region  11   p  (the length of the first substrate region  11   p  in the direction perpendicular to the interface F 1 ) is e.g. not less than 40 μm and not more than 60 μm. 
       FIGS. 9A and 9B  are schematic sectional views illustrating the electrostatic chuck according to the embodiment. 
       FIG. 9A  is an enlarged view of part of the first region  90   p  shown in  FIG. 8 .  FIG. 9B  is an enlarged view of part of the first substrate region  11   p  shown in  FIG. 8 . 
     As shown in  FIG. 9A , the first region  90   p  includes a plurality of grains g 1  (crystal grains). As shown in  FIG. 9B , the first substrate region  11   p  includes a plurality of grains g 2  (crystal grains). 
     The average grain size in the first region  90   p  (the average value of the diameter of the plurality of grains g 1 ) is different from the average grain size in the first substrate region  11   p  (the average value of the diameter of the plurality of grains g 2 ). 
     Thus, the average grain size in the first region  90   p  is different from the average grain size in the first substrate region  11   p . This can improve the coupling strength (interfacial strength) between the crystal grain of the porous part  90  and the crystal grain of the ceramic dielectric substrate  11  at the interface F 1 . For instance, this can suppress peeling of the porous part  90  from the ceramic dielectric substrate  11  and removal of crystal grains. 
     The average grain size can be the average value of the circle-equivalent diameter of the crystal grain in the cross-sectional image as shown in  FIG. 9A  and  FIG. 9B . The circle-equivalent diameter is the diameter of a circle having an area equal to the area of the target planar shape. 
     Preferably, the ceramic dielectric substrate  11  and the porous part  90  are integrated with each other. The porous part  90  is fixed to the ceramic dielectric substrate  11  by integration with the ceramic dielectric substrate  11 . This can improve the strength of the electrostatic chuck compared with the case of fixing the porous part  90  to the ceramic dielectric substrate  11  with e.g. adhesive. For instance, this can suppress the degradation of the electrostatic chuck due to e.g. corrosion or erosion of adhesive. 
     In this example, the average grain size in the first substrate region  11   p  is smaller than the average grain size in the first region  90   p . The small grain size in the first substrate region  11   p  can improve the coupling strength between the porous part  90  and the ceramic dielectric substrate at the interface between the porous part  90  and the ceramic dielectric substrate. The small grain size in the first substrate region can increase the strength of the ceramic dielectric substrate  11  and suppress the risk of e.g. cracks due to stress produced during manufacturing or processing. For instance, the average grain size in the first region  90   p  is not less than 3 μm and not more than 5 μm. For instance, the average grain size in the first substrate region  11   p  is not less than 0.5 μm and not more than 2 μm. The average grain size in the first substrate region  11   p  is not less than 1.1 times and not more than 5 times of the average grain size in the first region  90   p.    
     For instance, the average grain size in the first substrate region  11   p  is smaller than the average grain size in the second substrate region  11   q . In the first substrate region  11   p  provided in contact with the first region  90   p , preferably, the interfacial strength with the first region  90   p  is increased by interaction such as diffusion with the first region  90   p . On the other hand, in the second substrate region  11   q , preferably, the material of the ceramic dielectric substrate  11  develops its intrinsic characteristics. Thus, the average grain size in the first substrate region  11   p  is made smaller than the average grain size in the second substrate region  11   q . Accordingly, ensuring the interfacial strength in the first substrate region  11   p  can be made compatible with the characteristics of the ceramic dielectric substrate  11  in the second substrate region  11   q.    
     The average grain size in the first region  90   p  may be smaller than the average grain size in the first substrate region  11   p . This can improve the coupling strength between the porous part  90  and the ceramic dielectric substrate at the interface between the porous part  90  and the ceramic dielectric substrate. The small grain size in the first region  90   p  increases the strength of the porous part  90 . This can suppress removal of grains during processing and reduce particles. 
     Similarly to the foregoing, the average grain size in the first region  90   p  can be made smaller than the average grain size in the second substrate region  11   q . This can improve mechanical strength in the first region  90   p.    
     Referring again to  FIG. 2A , the structure of the electrostatic chuck  110  is further described. The electrostatic chuck  110  may further include a porous part  70  (a first porous part, a second porous part) as described previously. The porous part  70  does not include the plurality of sparse portions  94  and the plurality of dense portions  95  described in  FIGS. 3  to  7 . In this example, the porous part  70  is provided in the base plate and disposed to oppose the gas feed channel  53 . The porous part  70  can be provided between the porous part  90  and the gas feed channel  53  in the Z-direction, for instance. For instance, the porous part  70  is fitted into the ceramic dielectric substrate  11  side of the base plate  50 . As illustrated in  FIG. 2A , for instance, a countersink part  53   a  is provided on the ceramic dielectric substrate  11  side of the base plate  50 . The countersink part  53   a  is provided like a cylinder. The porous part  70  is fitted into the countersink part  53   a  by appropriately designing the inner diameter of the countersink part  53   a . As described later, the porous part  70  may be provided in the ceramic dielectric substrate  11 . 
     In this example, the upper surface  70 U of the porous part  70  is exposed to the upper surface  50 U of the base plate  50 . The upper surface  70 U of the porous part  70  is opposed to the lower surface  90 L of the porous part  90 . In this example, a space SP is formed between the upper surface  70 U of the porous part  70  and the lower surface  90 L of the porous part  90 . The first porous part can be any one of the porous part  90  and the porous part  70 . 
     The porous part  70  includes a porous region  71  including a plurality of pores (an example of a first porous region and a second porous region) and a dense region  72  (an example of a first dense region and a second dense region) denser than the porous region  71 . The porous region  71  is provided like a cylinder (e.g. circular cylinder) and fitted into the countersink part  53   a . The shape of the porous part  70  is preferably a circular cylinder, but is not limited to a circular cylinder. The porous part  70  is made of an insulative material. The material of the porous part  70  is e.g. Al 2 O 3 , Y 2 O 3 , ZrO 2 , MgO, SiC, AlN, or Si 3 N 4 . The material of the porous part  70  may be glass such as SiO 2 . The material of the porous part  70  may be e.g. Al 2 O 3 —TiO 2 , Al 2 O 3 —MgO, Al 2 O 3 —SiO 2 , Al 6 O 13 Si 2 , YAG, or ZrSiO 4 . 
     The porosity of the porous region  71  is e.g. not less than 20% and not more than 60%. The density of the porous region  71  is e.g. not less than 1.5 g/cm 3  and not more than 3.0 g/cm 3 . The gas such as He flowing from the gas feed channel  53  passes through a plurality of pores of the porous region  71  and is fed from the through hole  15  provided in the ceramic dielectric substrate  11  to the groove  14 . 
     The porous region  72  includes, for instance, a portion made of a ceramic insulating film. The ceramic insulating film is provided between the porous region  71  and the gas feed channel  53 . The ceramic insulating film  72  is denser than the porous region  71 . The porosity of the ceramic insulating film is e.g. 10% or less. The density of the ceramic insulating film is e.g. not less than 3.0 g/cm 3  and not more than 4.0 g/cm 3 . The ceramic insulating film is provided on the side surface of the porous part  70 . 
     The material of the ceramic insulating film is e.g. Al 2 O 3 , Y 2 O 3 , ZrO 2 , or MgO. The material of the ceramic insulating film may be e.g. Al 2 O 3 —TiO 2 , Al 2 O 3 —MgO, Al 2 O 3 —SiO 2 , Al 6 O 13 Si 2 , YAG, or ZrSiO 4 . 
     The ceramic insulating film can be formed by e.g. thermal spraying, PVD (physical vapor deposition), CVD, sol-gel method, or aerosol deposition method on the side surface of the porous part  70 . The film thickness of the film thickness is e.g. not less than 0.05 mm and not more than 0.5 mm. 
     The porosity of the ceramic dielectric substrate  11  is e.g. 1% or less. The density of the ceramic dielectric substrate  11  is e.g. 4.2 g/cm 3 . 
     The porosity in the ceramic dielectric substrate  11  and the porous part  70  is measured by a scanning electron microscope as described above. The density is measured based on JIS C 2141 5.4.3. 
     The porous part  70  is fitted in the countersink part  53   a  of the gas feed channel  53 . Then, the ceramic insulating film  72  is in contact with the base plate  50 . That is, the porous part  70  including the porous region  71  and the dense region  73  having high insulation performance is interposed between the through hole  15  for guiding the gas such as He to the groove  14  and the metallic base plate  50 . Use of such a porous part  70  can achieve higher insulation performance than in the case where only the porous region  71  is provided in the gas feed channel  53 . 
     The plurality of pores  71   p  provided in the porous part  70  are further dispersed in three dimensions than the plurality of pores  96  provided in the porous part  90 . The proportion of pores penetrating in the Z-direction can be made higher in the porous part  90  than in the porous part  70 . A higher breakdown voltage can be obtained by providing the porous part  70  including the plurality of pores  71   p  dispersed in three dimensions. This can suppress the occurrence of arc discharge effectively while smoothing the flow of the gas. The flow of gas can be smoothed by providing the first porous part  90  having a high proportion of pores penetrating in the Z-direction. As shown in  FIG. 2A , by providing the porous part  90  having a high proportion of pores penetrating in the Z-direction in the ceramic dielectric substrate  11 , the occurrence of arc discharge can be more effectively suppressed, for instance, even if a plasma density is high. 
     The average value of the plurality of pores provided in the porous part (the second porous part, the porous part  70  in  FIG. 2A ) provided in the base plate  50  can be made larger than the average value of the plurality of pores provided in the porous part (the first porous part, the porous part  90  in  FIG. 2A ) provided in the ceramic dielectric substrate  11 . Thus, the porous part having a large pore diameter is provided on the gas feed channel  53  side. This can smooth the flow of gas. The porous part  90  having a small pore diameter is provided on the suction target side. This can suppress the occurrence of arc discharge more effectively. 
     In the example where the porous part  70  is provided in the base plate  50  and the porous part  90  is provided in the ceramic dielectric substrate  11 , the average value of the diameter of the plurality of pores  71   p  provided in the porous part  70  can be made larger than the average value of the diameter of the plurality of pores  96  provided in the porous part  90 . Thus, the porous part  70  having a large diameter of the pore is provided. This can smooth the flow of the gas. The porous part  90  having a small diameter of the pore is provided on the suction target side. This can suppress the occurrence of arc discharge more effectively. 
     Variation in the diameter of the plurality of pores can be decreased. This can suppress arc discharge more effectively. 
       FIG. 10  is a schematic sectional view illustrating the porous part  70  of the electrostatic chuck according to the embodiment. 
       FIG. 10  is an enlarged view of a part of the cross section of the porous region  71 . 
     The plurality of pores  71   p  provided in the porous region  71  are dispersed in three dimensions in the X-direction, the Y-direction, and the Z-direction inside the porous region  71 . In other words, the porous region  71  has a three-dimensional network structure in which the pores  71   p  spread in the X-direction, the Y-direction, and the Z-direction. The plurality of pores  71   p  are dispersed in the porous region  71  e.g. randomly or uniformly in the porous part  70 . 
     The plurality of pores  71   p  are dispersed in three dimensions. Thus, parts of the plurality of pores  71   p  are also exposed to the surface of the porous region  71 . Accordingly, fine irregularities are formed at the surface of the porous region  71 . That is, the surface of the porous region  71  can be roughened. The surface roughness of the porous region  71  can facilitate forming e.g. the ceramic insulating film (the dense region  72 ) on the surface of the porous region  71 . For instance, this improves contact between the ceramic insulating film (the dense region  72 ) and the porous region  71 . Peeling of the ceramic insulating film (the dense region  72 ) can be suppressed. 
     The average value of the diameter of the plurality of pores  71   p  provided in the porous region  71  is larger than the average value of the diameter of the plurality of pores  96  provided in the porous region  91 . The diameter of the pore  71   p  is e.g. not less than 10 μm and not more than 50 μm. The porous region  91  having a small pore  96  diameter can control (limit) the flow rate of the gas flowing in the through hole  15 . This can suppress variation in the gas flow rate caused by the porous region  71 . The diameter of the pore  71   p  and the diameter of the pore  96  can be measured by a scanning electron microscope as described above. 
       FIG. 11  is a schematic sectional view illustrating a porous part  90  according to the other embodiment. 
       FIG. 11  illustrates the circumference of the porous part  90  as in  FIG. 2A . 
     In this example, the porous part  90  is provided in the ceramic dielectric substrate  11 . The porous part  70  is provided in the base plate  50 . That is, the porous part  90  is used for the first porous part. The porous part  70  is used for the second porous part. The porous part  90  may be provided in both of the ceramic dielectric substrate  11  and the base plate  50 . 
     In this example, a bonding member  61  (adhesive) is provided between the porous part  90  and the ceramic dielectric substrate  11 . The porous part  90  is bonded to the ceramic dielectric substrate  11  with the bonding member  61 . For instance, the bonding member  61  is provided between the side surface of the porous part  90  (the side surface  93   s  of the dense region  93 ) and the inner wall  15   w  of the through hole  15 . The porous part  90  and the ceramic dielectric substrate  11  do not need to be in contact with each other. 
     The bonding member  61  is e.g. a silicone adhesive. The bonding member  61  is e.g. an elastic member having elasticity. The elastic modulus of the bonding member  61  is e.g. lower than the elastic modulus of the dense region  93  of the porous part  90 , and lower than the elastic modulus of the ceramic dielectric substrate  11 . 
     In the structure in which the porous part  90  and the ceramic dielectric substrate  11  are bonded by the bonding member  61 , the bonding member  61  can be used as a cushioning material against the difference between the thermal contraction of the porous part  90  and the thermal contraction of the ceramic dielectric substrate  11 . 
       FIGS. 12A and 12B  are schematic sectional views illustrating the porous part  90  according to the other embodiment. 
     In the embodiment described above (see  FIG. 2 ), the porous par  90  is provided in the ceramic dielectric substrate  11 , and the porous part  70  is provided in the base plate  50 . 
     However, in the case where the porous part  90  is used, one the porous part provided in the base plate  50  or the porous part provided in the ceramic dielectric substrate  11  can be omitted. 
     For instance, in the example shown in  FIG. 12A , the porous part  90  is provided in the ceramic dielectric substrate  11 , and the gas feed channel  53  is provided in the base plate  50 . This can reduce a flow path resistance of gas such as He e.g. supplied to the porous part  90 . 
     In the example shown in  FIG. 12B , the hole part  15   b  is provided in the ceramic dielectric substrate  11 , and the porous part  90  is provided in base plate  50 . This can reduce a flow path resistance of gas such as He e.g. supplied to the porous part  90 . 
     As shown in  FIG. 12A , at least a part of the edge  53   b  of the opening on the ceramic dielectric substrate side of the gas feed channel can be configured to be a curve. For instance, so called “R chamfer” can be made for the edge  53   b  of the opening of the gas feed channel  53 . In this case, the edge  53   b  of the opening of the gas feed channel  53  can be configured to be a curve with a radius of about 0.2 millimeters (mm). 
     As described previously, the base plate  50  is formed of a metal such as aluminum. Therefore, when the edge of the opening of the gas feed channel  53  is sharp, the electric field concentration is easy to occur, and the arc discharge may easily occur. 
     In the embodiment, at least a part of the edge  53   b  of the opening of the gas feed channel  53  is configured to be a curved. This can suppress the electric field concentration, and further can reduce the arc discharge. 
       FIGS. 13A to 13D  are schematic sectional views illustrating porous parts  90   a ,  70   a  according to the other embodiment. 
       FIGS. 14A to 14C  are schematic sectional views illustrating porous parts  90   a ,  90   b  according to the other embodiment. 
       FIG. 13A  shows the example of the case where the ceramic dielectric substrate  11  is provided with the porous part  90   a  modified from the dense region  93  of the porous part  90 , and the base plate  50  is provided with the porous part  70   a  modified from the dense region  72  of the porous part  70 .  FIG. 14A  is the example of the case where the ceramic dielectric substrate  11  and the base plate  50  are provided with the porous part  90   a  modified from the dense region  93  of the porous part  90  and the porous part  70   b  modified from the dense region  72  of the porous part  70 , respectively. 
     As shown in  FIG. 13A ,  FIG. 13B , and  FIG. 14A , in the porous part  90   s  provided in the ceramic dielectric substrate  11 , the porous region  91  further includes a dense part  92   a . That is, in the porous part  90   a , the dense part  92   a  is further added to the porous part  90  described above. 
     As shown in  FIG. 13A  and  FIG. 13B , the dense part  92   a  can present a plate shape (for instance, disk shape). As shown in  FIG. 14A , the dense part  92   a  can also present a columnar shape (for instance, cylindrical shape). A material of the dense part  92   a  can be, for instance, similar to a material of the dense region  93  described above. The dense part  92   a  is denser than the porous region  91 . The dense part  92   a  may have approximately same denseness as the dense region  93 . As projected on a plane (XY-lane) perpendicular to the Z-direction, the dense part  92   a  overlaps the hole part  15   b . It is preferable that the porous region  91  is configured not to overlap the hole part  15   b . In such configuration, a generated current tries to flow around the dense part  92   a . For that reason, a distance (conduction path) where the current flows can be longer, and thus an electron is not easy to be accelerated, and further the arc discharge can be suppressed from occurring. 
     As projected on a plane (XY-lane) perpendicular to the Z-direction, it is preferable that the dimension of the dense part  92   a  is the same as the dimension of the hole part  15   b , or the dimension of the dense part  92   a  is larger than the dimension of the hole part  15   b . This can introduce the current which flows the interior of the hole part  15   b  into the dense part  92   a . Thus, the distance (conduction path) where the current flows can be made long effectively. 
     In this example, as projected on a plane (XY-lane) perpendicular to the Z-direction, the porous region  91  is provided around the dense part  92   a . While improving the resistance to the arc discharge by arranging the dense part  92   a  at a position opposing to the hole part  15   b , the surrounding is the porous region  91 , and thus sufficient gas flow can be ensured. That is, reducing the arc discharge and smoothing the gas flow can be compatible. 
     As shown in  FIG. 13A , the length along the Z-direction of the dense part  92   a  may be smaller than the length along the Z-direction of the porous part  90   a , and as shown in  FIG. 14A , may be substantially the same as the length along the Z-direction of the porous part  90   a . If the length along the Z-direction of the dense part  92   a  is made longer, the occurrence of arc discharge can be suppressed more effectively. If the length along the Z-direction of the dense part  92   a  is made smaller than the length along the Z-direction of the porous part  90   a , the gas flow can be smoothed. 
     The dense part  92   a  may be configured by a dense body substantially not including pores, and if being denser than the porous region  91 , the dense part  92   a  may be configured to include a plurality of pores. When the dense part  92   a  includes the plurality of pores, the diameter of the pores is preferably made smaller than the diameter of pores included in the porous region  91 . Porosity (percent: %) of the dense part  92   a  can be lower than porosity (%) of the porous region  91 . Thus, density (gram/cubic centimeter: g/cm 3 ) of the dense part  92   a  can be higher than density (g/cm 3 ) of the porous region  91 . The porosity of the dense part  92   a  can be, for instance, the same as the porosity of the dense region  93  described above. 
     Here, arc discharge often occurs by a current flows from the ceramic dielectric substrate  11  side toward the base plate  50  side in the interior of the hole part  15   b . Thus, if the dense part  92   a  with low porosity is provided near the hole part  15   b , as shown in  FIG. 13A  and  FIG. 14A , a current  200  tries to flow around the dense part  92   a . For that reason, a distance (conduction path) where a current  200  flows can be made longer, and thus an electron is not easy to be accelerated, and further the arc discharge can be suppressed from occurring. 
     As shown in  FIG. 13A , for instance, for the porous part  70  provided in the base plate  50 , the porous part  70   a  including the porous region  71  further including a dense part  92   b  can be used. 
     As shown in  FIG. 14A , the porous part  90   a  can be provided in the ceramic dielectric substrate  11 , and the porous part  70   b  can be also provided in the base pate  50 . In the porous part  70   b , the porous region  71  further includes the dense part  92   b . That is, in the porous part  70   b , the dense part  92   b  is further added to the porous part  70  described above. 
     That is, the dense part  92   b  can also be further added to the porous part  70  or the porous part  90  provided in the base plate  50 . 
     At least one dense part  92   b  can be provided. As shown in  FIG. 13C  and  FIG. 14B , a plurality of dense parts  92   b  presenting a plate shape (for instance, disk shape) or a columnar shape (for instance, cylindrical column shape) can also be provided. As shown in  FIG. 13D  and  FIG. 14C , the dense part  92   b  presenting a ring shape (for instance, circular ring shape) or a tube shape (for instance, cylindrical shape) can also be provided. A material, density, porosity e.g. of the dense part  92   b  can be the same as the dense part  92   a.    
     As projected on a plane (XY-plane) perpendicular to the Z-direction, it is preferable that at least a part of the dense part (e.g. dense part  92   b ) included in the porous part provided in the base plate  50  overlaps the dense part (e.g. dense part  92   a ) included in the porous part provided in the ceramic dielectric substrate  11 . In such configuration, for instance, when the current which flows around the dense part  92   a  in the porous part  90   a  (porous part on the ceramic dielectric substrate  11  side) flows through the porous parts  70 ,  90   b  (the porous part on the base plate  50  side) provided with the dense part  92   b , the current does not flow in the porous regions (e.g. porous regions  71 ,  91 ) of the porous part provided on the base plate  50  side, and further tries to flow around the dense part  92   b . For that reason, the distance (conduction path) where the current flows can be made longer, and thus an electron is not easy to be accelerated, and further arc discharge can be suppressed from occurring. 
       FIGS. 15A and 15B  are schematic sectional views illustrating the porous part according to the other embodiment. 
     As shown in  FIGS. 15A and 15B , as projected on a plane (XY-plane) perpendicular to the Z-direction, the dense part  92   a  and the dense part  92   b  are configured to overlap. As projected on a plane (XY-plane) perpendicular to the Z-direction, if a gap between the dense part  92   a  and the dense part  92   b  is small, a current can be suppressed from flowing between the dense part  92   a  and the dense part  92   b . Thus, as long as the current can be suppressed from flowing between the dense part  92   a  and the dense part  92   b , the gap can be provided between the dense part  92   a  and the dense part  92   b.    
     In this way, the current which flows in the porous part  90   a  can be suppressed from flowing in the porous part  70   a  without through the dense part  92   b . Thus, the distance (conduction path) where the current flows can be made longer effectively. 
     As shown in  FIGS. 15A and 15B , as projected on a plane (XY-plane) perpendicular to the Z-direction, it is preferable that the dense part  92   b  overlap the dense region  93 . As projected on a plane (XY-plane) perpendicular to the Z-direction, the dense part  92   b  may contact the dense region  93 . In this way, the distance (conduction path) where the current flows can be made longer, and thus an electron is not easy to be accelerated, and further arc discharge can be suppressed from occurring. 
       FIG. 16  is a schematic sectional view illustrating an electrostatic chuck according to the other embodiment. 
       FIGS. 17A and 17B  correspond to enlarged views of the region C shown in  FIG. 16 . 
     As shown in  FIGS. 17A and 17B , an electrostatic chuck  110   a  includes a ceramic dielectric substrate  11   c , and the base plate  50 . That is, the ceramic dielectric substrate  11   c  is not provided with the porous part (porous part  70  or porous part  90 ). 
     The ceramic dielectric substrate  11   c  is directly provided with a plurality of holes  16 . The plurality of holes  16  can be formed in the ceramic dielectric substrate  11   c , for instance, by laser irradiation or ultrasonic processing e.g. In this example, one end of the plurality of holes  16  is positioned on a surface  14   a  of the groove  14 . Other end of the plurality of holes  16  is positioned on the second major surface  11   b  of the ceramic dielectric substrate  11   c . That is, the plurality of holes  16  pierces the ceramic dielectric substrate  11   c  in the Z-direction. 
     As shown in  FIGS. 17A and 17B , the base plate  50  can be provided with the porous part (e.g. porous part  70   a ). The base plate  50  may be provided with the porous part  90   b . As illustrated in  FIG. 12A , the base plate  50  may be provided with the gas feed channel  53  without being provided with the porous part  70  or the porous part  90 . 
     As shown in  FIGS. 17A and 17B , an upper surface  70 U of the porous part  70  (or an upper surface  90 U of the porous part  90 ) provided on the base plate  50  and the second major surface  11   b  of the ceramic dielectric substrate  11   c  may not contact. As before, at least a part of the edge  53   b  of the opening on the ceramic dielectric substrate  11   c  side of the gas feed channel  53  can be configured to be a curve. 
     If the plurality of holes  16  are provided in the ceramic dielectric substrate  11   c , while ensuring the flow rate the gas supplied between a back of the target W placed on the electrostatic chuck  110   a  and the first major surface  11   a  including the groove  14 , the resistance to arc discharge can be improved. 
     The length along the Z-direction of the dense part  92   b  can be smaller than the length along the Z-direction of the porous parts  70   a ,  90   b . The length along the Z-direction of the dense part  92   b  can also be substantially the same as the length along the Z-direction of the porous parts  70   a ,  90   b . If the length along the Z-direction of the dense part  92   b  is made short, the gas flow can be smoothed. The length along the Z-direction of the dense part  92   b  is made long, the occurrence of arc discharge can be suppressed more effectively. 
     As projected on a plane (XY-plane) perpendicular to the Z-direction, at least one of the plurality of holes  16  can be configured to overlap the dense part  92   b . A material, density, porosity e.g. of the plurality of the dense parts  92   b  are as described above, for instance. 
     As shown in  FIG. 17A , the porous part  70   a  including the plurality of dense parts  92   b  may be provided in the base plate  50 . As shown in  FIG. 17B , the porous part  90   b  including the plurality of dense parts  92   b  may be provided in the base plate  50 . 
       FIG. 18  is a schematic sectional view illustrating a plurality of holes  16   h  according to the other embodiment. 
     The plurality of holes  16   h  can be formed in the ceramic dielectric substrate  11   c  by laser irradiation or ultrasonic processing. 
     As shown in  FIG. 18 , at least one of the plurality of holes  16   h  provided in the ceramic dielectric substrate  11   c  can include a first portion  16   h   1  opening to the groove  14  and a second portion  16   h   2  opening to the second major surface  11   b . In the X-direction or the Y-direction, a dimension of the first portion  16   h   1  can be smaller than a dimension of the second portion  16   h   2 . In the X-direction or the Y-direction, at least one of the plurality of holes  16   h  can be configured to have an opening dimension D 4  on the surface  14   a  side of the groove  14  smaller than an opening dimension D 3  on the base plate  50  side. In  FIG. 18 , the holes  16   h  having a stepped structure are illustrated, however the holes  16  can be configured to have a tapered structure. For instance, the opening dimension D 4  can be 0.01 millimeters (mm)˜0.1 millimeters (mm) in a diameter. For instance, the opening dimension D 3  can be about 0.15 millimeters (mm)˜0.2 millimeters (mm) in a diameter. If the opening dimension D 4  is smaller than the opening dimension D 3 , the occurrence of arc discharge can be suppressed effectively. 
     An aspect ratio of the holes  16   h  can be, for instance, 3˜60. In calculating the aspect ratio, “vertical” is taken, for instance, as a length in the Z-direction of the holes  16   h  in  FIG. 18 , and “horizontal” is taken as an average length of a length in the X-direction of the holes  16   h  in the upper surface (surface  14   a ) of the holes  16   h  and a length in the X-direction of the holes  16   h  in the lower surface (second major surface  11   b ) of the holes  16   h . The measurement of the length in the X-direction of the holes  16   h  can be performed by using an optical microscopy, a digital microscope e.g. such as a laser microscopy and a factory microscopy e.g. 
     The opening dimension D 4  can be smaller than a length L 1  of the sparse portion  94   a  illustrated in  FIG. 4  (a length L 4  of the sparse portion  94   b , a length L 5  of the sparse portion  94   c ). 
       FIGS. 19A and 19B  are schematic sectional views illustrating a shape of the opening portion of the hole  16 . 
       FIG. 19B  corresponds to an enlarged view of a region D shown in  FIG. 19A . In  FIG. 19A , the hole  16  has not a portion with the opening dimension of D 3  shown in  FIG. 18 . That is, the opening dimension of the hole  16  corresponds to the opening dimension D 4  shown in  FIG. 18 . 
     As shown in  FIGS. 19A and 19B , an edge  16   i  of the opening on the first major surface  11   a  side (the surface  14   a  side of the groove  14 ) of the hole  16  can be inclined more smoothly than an edge  16   j  of the opening on the second major surface  11   b  side of the hole  16 . In this example, in at least one of the plurality of holes  16 , when an angle between the edge  16   i  of the opening on the groove  14  side of the hole  16  and the surface  14   a  on the second major surface  11   b  side of the groove  14  is taken a, and an angle between the edge  16   j  of the opening on the second major surface  11   b  side of the hole  16  and the second major surface  11   b  is taken as β, “α&lt;β” is obtained. This can suppress the electric field concentration, and further the arc discharge can be reduced. In this example, the edge  16   i  is configured to be a straight line. However, the edge  16   i  may be configure to be a curve, and may be configured to be a straight line and a curve. When the edge  16   i  and the edge  16   j  are configured to be a curve, a curvature radius of the edge  16   i  can be larger than a curvature radius of the edge  16   j . When the edge  16   i  and the edge  16   j  are configured to be a straight line and a curvature, it is sufficient that at least the relationship between the straight line portions and the relationship between the curve portions satisfies the relationship described above. 
     When the edge  16   i  is inclined more gradually than the edge  16   j , the occurrence of chipping or the electric field concentration can be suppressed. Thus, the occurrence of arc discharge can be more effectively suppressed. 
     The shape of the opening portion of the hole  16  is illustrated as an example, however it is much the same for the hole  16  having a stepped structure or a tapered structure. 
       FIG. 20  is a schematic sectional view illustrating the electrostatic chuck according to the other embodiment. 
       FIG. 20  corresponds to an enlarged view of the region C shown in  FIG. 16 . 
     Each of the plurality of holes  16  illustrated in  FIG. 17A  extends substantially in the Z-direction. On the contrary, at least one of the plurality of holes  16  illustrated in  FIG. 20  can be inclined to the Z-direction. If at least one of the plurality of holes  16  extends in a direction inclined to the Z-direction, it is considered that electrons are less likely to be accelerated in current flow in the hole  16 . Thus, the occurrence of arc discharge can be more effectively suppressed. According to knowledge obtained by the inventors, if the angle θ inclined to the Z-direction is not less than 5° and not more than 30°, preferably not less than 5° and not more than 15°, the occurrence of arc discharge can be suppressed without making the diameter of the hole  16  small. 
     The case of the hole  16  is illustrated as an example, however it is much the same for the hole  16   h  having the stepped structure or the tapered structure. 
     The hole  16  inclined to the Z-direction can be directly formed in the ceramic dielectric substrate  11   c  by laser irradiation or ultrasonic processing. Therefore, the region provided with at least one hole  16  inclined to the Z-direction includes the same material as the ceramic dielectric substrate  11 . 
       FIG. 21  is a schematic sectional view illustrating the electrostatic chuck according to the other embodiment. 
       FIG. 21  corresponds to an enlarged view of the region C shown in  FIG. 16 . 
     As shown in  FIG. 21 , the porous region  91  of the porous part  90   b  further includes the dense part  92   b . That is, the porous part  90   b  further adds the dense part  92   b  to the porous part  90  described above. That is, the dense part  92   b  can also be further added to the porous part  60  or the porous part  90  provided in the base plate  50 . 
     As projected on a plane (XY-plane) perpendicular to the Z-direction, at least one of the openings on the dense part  92   b  side of the plurality of holes  16  can be configured to overlap the dense part  92   b . A material, density, porosity e.g. of the plurality of dense parts  92   b  are as described above, for instance. 
       FIG. 22  is a schematic sectional view illustrating an electrostatic chuck according to the other embodiment. 
       FIG. 23  corresponds to an enlarged view of the region E shown in  FIG. 22 . 
       FIG. 24  is an enlarged view showing the other embodiment of the region E shown in  FIG. 22 . 
     As shown in  FIG. 22 ,  FIG. 23 , and  FIG. 24 , an electrostatic chuck  11   b  includes a ceramic dielectric substrate  11   d , and the base plate  50 . The ceramic dielectric substrate  11   d  is provided with the porous part  90   b  or the porous part  90   a.    
     The ceramic dielectric substrate  11   d  is provided with the plurality of holes  16 . The plurality of holes  16  can be formed in the ceramic dielectric substrate  11   d , for instance, by laser irradiation or ultrasonic processing e.g. In this example, one end of the plurality of holes  16  is positioned on the surface  14   a  of the groove  14 . The other end of the plurality of holes  16  is positioned on a bottom surface of the hole part  15   c . That is, the plurality of holes  16  pierces the ceramic dielectric substrate  11   d  in the Z-direction. 
     As shown in  FIG. 23 , as projected on a plane (XY-plane) perpendicular to the Z-direction, at least one of the plurality of holes  16  can be configured to overlap the dense part  92   b . A material, density, porosity e.g. of the dense part  92   a  are as described above, for instance. 
     (Processing Apparatus) 
       FIG. 25  is a schematic view illustrating a processing apparatus  200  according to the embodiment. 
     As shown in  FIG. 25 , the processing apparatus  200  can be provided with the electrostatic chuck  110 , a power supply  210 , a medium supplier  220 , and a supplier  230 . 
     The power supply  210  is electrically connected to the electrode  12  provided in the electrostatic chuck  110 . The power supply  210  can be, for instance, a direct current power supply. The power supply  210  applies a predetermined voltage to the electrode  12 . The power supply  210  can also be provided with a switch for switching between voltage application and voltage application stop. 
     The medium supplier  220  is connected to the input channel  51  and the output channel  52 . The medium supplier  220  can supply, for instance, liquid serving as a cooling medium or a heat insulation medium. 
     The medium supplier  220  includes, for instance, a storage part  221 , a control valve  222 , and an evacuation part  223 . 
     The storage part  221  can be, for instance, a tank storing the liquid, or a factory pipe e.g. The storage part  221  can be provided with a cooling device or heating device for controlling the liquid temperature. The storage part  221  can also include a pump e.g. for pumping the liquid. 
     The control valve  222  is connected between the input channel  51  and the storage part  221 . The control valve  222  can control at least one of a flow rate or a pressure of the liquid. The control valve  222  can switch liquid supply and supply stop. 
     The evacuation part  223  is connected to the output channel  52 . The evacuation part  223  can be a tank or a drain pipe to recover the liquid evacuated from the output channel  52 . The evacuation part  223  is not always necessary, and the liquid evacuated from the output channel  52  may be supplied to the storage part  221 . This can circulate the cooling medium or the heat insulation medium, and thus resource saving is possible. 
     The supplier  230  includes a gas supplier  231  and a gas controller  232 . 
     The gas supplier  231  can be a high pressure cylinder storing a gas such as He e.g. or a factory pipe e.g. The case where one gas supplier  231  is provided is illustrated, however a plurality of gas suppliers  231  may be provided. 
     The gas controller  232  is connected between the plurality of gas feed channels  53  and the gas supplier  231 . The gas controller  232  can control at least one of a flow rate or a pressure of the gas. The gas controller  232  can be configured to further include function for switching gas supply and supply stop. The gas controller  232  can be, for instance, a mass flow controller or mass flow meter e.g. 
     As shown in  FIG. 25 , the gas controller  232  can be provided in a plurality. For instance, the gas controller  232  can be provided every plurality of regions of the first major surface  11   a . This can perform control of the supplied gas every plurality of regions. In this case, the gas controller  232  can be provided every plurality of gas feed channels  53 . This can perform control of the gas in the plurality of regions more precisely. The case where the plurality of gas controllers  232  are provided is illustrated, however one gas controller is sufficient as long as the gas controller  232  is possible to control independently the gas supply in the plurality of gas supply systems. 
     Here, there is a vacuum chuck and a mechanical chuck e.g. as means for holding the target W. However, the vacuum chuck cannot be used in environment reduced from atmosphere. When the mechanical chuck is used, the target W may be damaged and particles may occur. Therefore, for instance, an electrostatic chuck is used for a processing apparatus used in a semiconductor manufacturing process e.g. 
     In the processing apparatus like this, a processing space is needed to be isolated from external environment. Thus, the processing apparatus  200  can further include a chamber  240 . The chamber  240  can be configured to have an airtight structure possible to maintain the environment reduced from the atmosphere. 
     The processing apparatus  200  can include a plurality of lift pins and a drive device lifting the plurality of lift pins. When the target W is received from a transfer device or the target W is handed over to the transfer device, the lift pin is raised by the drive device and protrudes from the first major surface  11   a . When the target W received from the transfer device is placed on the first major surface  11   a , the lift pin is lowered by the drive device and is stored in the ceramic dielectric substrate  11 . 
     The processing apparatus  200  can be provided with various devices depending on processing contents. For instance, a vacuum pump evacuating the interior of the chamber  240  can be provided. A plasma generation device generating plasma in the interior of the chamber  240  can be provided. A process gas supplier supplying the process gas into the interior of the chamber  240  can be provided. A heater heating the target W and the process gas in the interior of the chamber  2340  can also be provided. The devices provided in the processing apparatus  200  are not limited to the illustration. Well known arts can be applied to the devices provided in the processing apparatus  200 , and thus detailed description will be omitted. 
     The embodiments of the invention have been described above. However, the invention is not limited to the above description. For instance, the electrostatic chuck  110  has been illustrated with reference to the configuration based on the Coulomb force. However, the electrostatic chuck  110  is also applicable to the configuration based on the Johnsen-Rahbek force. Those skilled in the art can appropriately modify the above embodiments, and such modifications are also encompassed within the scope of the invention as long as they include the features of the invention. Furthermore, various components in the above embodiments can be combined with each other as long as technically feasible. Such combinations are also encompassed within the scope of the invention as long as they include the features of the invention.