Patent Publication Number: US-2022223388-A1

Title: Exhaust ring assembly and plasma processing apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2021-002316, filed on Jan. 8, 2021, the entire contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to an exhaust ring assembly and a plasma processing apparatus. 
     BACKGROUND 
     Patent Document 1 discloses a parallel type exhaust ring in which two members are arranged to overlap each other in the horizontal direction. Patent Document 2 discloses a cylindrical exhaust ring in which two members are arranged to overlap each other in the vertical direction. 
     Each of the exhaust rings has a double structure in which two members having exhaust holes are arranged in an overlapping manner. In this case, gas passes through an exhaust hole in the member located at the upstream side with respect to a gas flow direction. Subsequently, the gas flow direction is changed to be approximately vertical, and is further changed to be approximately vertical when passing through an exhaust hole in the member located at the downstream side so that the gas is exhausted. 
     PRIOR ART DOCUMENTS 
     Patent Documents 
     Patent Document 1: Japanese Laid-Open Patent Publication No. 2004-006574 
     Patent Document 2: Japanese Laid-Open Patent Publication No. 2004-327767 
     Incidentally, in the exhaust ring, when the opening ratio is increased by enlarging the exhaust hole, the exhaust efficiency is improved, but plasma is more likely to leak from the plasma processing space to the exhaust space. Therefore, there is a trade-off relationship between the exhaust efficiency of the exhaust ring and the confinement effect of the plasma. 
     SUMMARY 
     According to one embodiment of the present disclosure, an exhaust ring assembly disposed around a substrate support includes: a first annular member having a plurality of first exhaust holes and a plurality of first rod-shaped portions alternately arranged in a circumferential direction, each of the plurality of first exhaust holes extending in a radial direction and each of the plurality of first rod-shaped portions extending in the radial direction; and a second annular member disposed below the first annular member and having a plurality of second exhaust holes and a plurality of second rod-shaped portions alternately arranged in the circumferential direction, each of the plurality of second exhaust holes extending in the radial direction and each of the plurality of second rod-shaped portions extending in the radial direction, wherein the plurality of first rod-shaped portions and the plurality of second rod-shaped portions do not overlap each other when viewed from above, and at least one of each of the plurality of first rod-shaped portions and each of the plurality of second rod-shaped portions has an upwardly-tapered shape. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a portion of the specification, illustrate embodiments of the present disclosure, and together with the general description given above and the detailed description of the embodiments given below, serve to explain the principles of the present disclosure. 
         FIG. 1  is a view illustrating an example of a plasma processing system according to an embodiment. 
         FIG. 2  is a vertical cross-sectional view illustrating an example of a plasma processing apparatus according to an embodiment. 
         FIGS. 3A to 3C  are views illustrating examples of exhaust rings according to an embodiment. 
         FIGS. 4A and 4B  are views illustrating an example of an exhaust ring according to an embodiment. 
         FIGS. 5A and 5B  are a top view and a cross-sectional view of a portion of an exhaust ring according to an embodiment. 
         FIGS. 6A to 6C  are views illustrating examples of flows of gas passing through exhaust rings according to an embodiment and a comparative example. 
         FIGS. 7A and 7B  are views illustrating examples of flows of gas passing through exhaust rings according to another comparative example. 
         FIGS. 8A to 8C  are views illustrating an example of a method of thermal spraying an exhaust ring according to an embodiment in comparison with a comparative example. 
         FIGS. 9A to 9C  are views illustrating an example of a method of thermal spraying an exhaust ring according to an embodiment in comparison with a comparative example. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings. In the drawings, there are cases where the same components are designated by like reference numerals with the repeated descriptions thereof omitted. In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the present disclosure. However, it will be apparent to one of ordinary skill in the art that the present disclosure may be practiced without these specific details. In other instances, well-known methods, procedures, systems, and components have not been described in detail so as not to unnecessarily obscure aspects of the various embodiments. 
     [Plasma Processing System] 
     First, a plasma processing system according to an embodiment will be described with reference to  FIG. 1 .  FIG. 1  is a view illustrating an example of the plasma processing system according to an embodiment. In an embodiment, the plasma processing system includes a plasma processing apparatus  1  and a controller  2 . The plasma processing apparatus  1  includes a plasma processing chamber  10 , a substrate support  11 , and a plasma generator  12 . The plasma processing chamber  10  includes a plasma processing space. In addition, the plasma processing chamber  10  includes at least one gas supply port configured to supply at least one processing gas to the plasma processing space, and at least one gas discharge port configured to discharge gas from the plasma processing space. The gas supply port is connected to a gas supplier  20  to be described later, and the gas discharge port is connected to an exhaust system  40  to be described later. The substrate support  11  is arranged in the plasma processing space and has a substrate support surface for supporting a substrate. 
     The plasma generator  12  is configured to generate plasma from the at least one processing gas supplied into the plasma processing space. The plasma formed in the plasma processing space may be capacitively coupled plasma (CCP), inductively coupled plasma (ICP), electron-cyclotron-resonance (ECR) plasma, helicon wave plasma (HWP), surface wave plasma (SWP), or the like. In addition, various types of plasma generators including an alternating current (AC) plasma generator and a direct current (DC) plasma generator may be used. In an embodiment, an AC signal (AC power) used in the AC plasma generator has a frequency in the range of 100 kHz to 10 GHz. Therefore, the AC signal includes a radio frequency (RF) signal and a microwave signal. In an embodiment, the RF signal has a frequency in the range of 200 kHz to 150 MHz. 
     The controller  2  processes computer-executable commands that cause the plasma processing apparatus  1  to execute various processes described in the present disclosure. The controller  2  may be configured to control each element of the plasma processing apparatus  1  to perform various steps described herein. In an embodiment, a portion or all of the controller  2  may be included in the plasma processing apparatus  1 . The controller  2  may include, for example, a computer  2   a . The computer  2   a  may include, for example, a processing part (a central processing unit (CPU))  2   a   1 , a storage part  2   a   2 , and a communication interface  2   a   3 . The processing part  2   a   1  may be configured to perform various control operations based on a program stored in the storage part  2   a   2 . The storage part  2   a   2  may include a random access memory (RAM), a read only memory (ROM), a hard disk drive (HDD), a solid state drive (SSD), or a combination thereof. The communication interface  2   a   3  may communicate with the plasma processing apparatus  1  via a communication line such as a local area network (LAN). 
     [Plasma Processing Apparatus] 
     Next, a configuration example of a capacitively coupled plasma processing apparatus as an example of the plasma processing apparatus  1  will be described with reference to  FIG. 2 .  FIG. 2  is a vertical cross-sectional view illustrating an example of the plasma processing apparatus  1  according to an embodiment. 
     The plasma processing apparatus  1  includes a plasma processing chamber  10 , a gas supplier  20 , a power supply  30 , and an exhaust system  40 . In addition, the plasma processing apparatus  1  includes a substrate support  11  and a gas introduction part. The gas introduction part is configured to introduce the at least one processing gas into the plasma processing chamber  10 . The gas introduction part includes a shower head  13 . The substrate support  11  is arranged in the plasma processing chamber  10 . The shower head  13  is arranged above the substrate support  11 . In an embodiment, the shower head  13  constitutes at least a portion of the ceiling of the plasma processing chamber  10 . The plasma processing chamber  10  includes a plasma processing space  10   s  defined by the shower head  13 , a sidewall  10   a  of the plasma processing chamber  10 , and the substrate support  11 . The sidewall  10   a  is grounded. The shower head  13  and the substrate support  11  are electrically insulated from a housing of the plasma processing chamber  10 . 
     The substrate support  11  includes a main body  111  and a ring assembly  112 . The main body  111  includes a central region (a substrate support surface)  111   a  for supporting a substrate (wafer) W and an annular region (a ring support surface)  111   b  for supporting the ring assembly  112 . The annular region  111   b  of the main body  111  surrounds the central region  111   a  of the main body  111  in a plan view. The substrate W is placed on the central region  111   a  of the main body  111 , and the ring assembly  112  is disposed on the annular region  111   b  of the main body  111  to surround the substrate W on the central region  111   a  of the main body  111 . In an embodiment, the main body  111  includes a base and an electrostatic chuck. The base includes a conductive member. The conductive member of the base functions as a lower electrode. The electrostatic chuck is placed on the base. The top surface of the electrostatic chuck has the substrate support surface  111   a . The ring assembly  112  includes one or more annular members. At least one of the one or more annular members is an edge ring. Although not illustrated, the substrate support  11  may include a temperature regulation module configured to regulate a temperature of at least one of the electrostatic chuck, the ring assembly  112 , and the substrate to a target temperature. The temperature regulation module may include a heater, a heat transfer medium, a flow path, or a combination thereof. A heat transfer fluid, such as brine or gas, flows through the flow path. The substrate support  11  may include a heat transfer gas supplier configured to supply a heat transfer gas to a space between a rear surface of the substrate W and the substrate support surface  111   a.    
     The shower head  13  is configured to introduce at least one processing gas from the gas supplier  20  into the plasma processing space  10   s . The shower head  13  includes at least one gas supply port  13   a , at least one gas diffusion chamber  13   b , and a plurality of gas inlet ports  13   c . The processing gas supplied to the gas supply port  13   a  passes through the gas diffusion chamber  13   b  and is introduced into the plasma processing space  10   s  from the plurality of gas inlet ports  13   c . In addition, the shower head  13  includes a conductive member. The conductive member of the shower head  13  functions as an upper electrode. In addition to the shower head  13 , the gas introduction part may include one or more side gas injectors SGI installed in one or more openings formed in the sidewall  10   a.    
     The gas supplier  20  may include at least one gas source  21  and at least one flow rate controller  22 . In an embodiment, the gas supplier  20  is configured to supply at least one processing gas from a corresponding gas source  21  to the shower head  13  via a corresponding flow rate controller  22 . Each flow rate controller  22  may include, for example, a mass flow controller or a pressure-controlled flow rate controller. The gas supplier  20  may include at least one flow rate modulation device configured to modulate or pulse the flow rate of the at least one processing gas. 
     The power supply  30  includes an RF power supply  31  coupled to the plasma processing chamber  10  via at least one impedance matching circuit. The RF power supply  31  is configured to supply at least one RF signal (RF power) such as a source RF signal and a bias RF signal to the conductive member of the substrate support  11  and/or the conductive member of the shower head  13 . As a result, plasma is formed from the at least one processing gas supplied to the plasma processing space  10   s . Therefore, the RF power supply  31  may function as at least a portion of the plasma generator  12 . By supplying the bias RF signal to the conductive member of the substrate support  11 , a bias potential is generated in the substrate W, and an ionic component in the formed plasma can be drawn into the substrate W. 
     In an embodiment, the RF power supply  31  includes a first RF generator  31   a  and a second RF generator  31   b . The first RF generator  31   a  is coupled to the conductive member of the substrate support  11  and/or the conductive member of the shower head  13  via at least one impedance matching circuit, and is configured to generate a source RF signal (source RF power) for plasma generation. In an embodiment, the source RF signal has a frequency in the range of 13 MHz to 150 MHz. In an embodiment, the first RF generator  31   a  may be configured to generate multiple source RF signals having different frequencies. The generated one or more source RF signals are supplied to the conductive member of the substrate support  11  and/or the conductive member of the shower head  13 . The second RF generator  31   b  is coupled to the conductive member of the substrate support  11  via at least one impedance matching circuit, and is configured to generate a bias RF signal (bias RF power). In an embodiment, the bias RF signal has a lower frequency than the source RF signal. In an embodiment, the bias RF signal has a frequency in the range of 400 kHz to 13.56 MHz. In an embodiment, the second RF generator  31   b  may be configured to generate multiple bias RF signals having different frequencies. The generated one or more bias RF signals are supplied to the conductive member of the substrate support  11 . In addition, in various embodiments, at least one of the source RF signal and the bias RF signal may be pulsed. 
     The power supply  30  may include a DC power supply  32  coupled to the plasma processing chamber  10 . The DC power supply  32  includes a first DC generator  32   a  and a second DC generator  32   b . In an embodiment, the first DC generator  32   a  is connected to the conductive member of the substrate support  11  and is configured to generate a first DC signal. The generated first DC signal is applied to the conductive member of the substrate support  11 . In an embodiment, the first DC signal may be applied to another electrode such as an electrode in an electrostatic chuck. In an embodiment, the second DC generator  32   b  is connected to the conductive member of the shower head  13  and is configured to generate a second DC signal. The generated second DC signal is applied to the conductive member of the shower head  13 . In various embodiments, the first and second DC signals may be pulsed. The first and second DC generators  32   a  and  32   b  may be provided in addition to the RF power supply  31 , or the first DC generator  32   a  may be provided in place of the second RF generator  31   b.    
     The exhaust system  40  may be connected to, for example, a gas discharge port  10   e  provided in the bottom portion of the plasma processing chamber  10 . The exhaust system  40  may include a pressure regulation valve and a vacuum pump. By the pressure regulation valve, an internal pressure of the plasma processing space  10   s  is regulated. The vacuum pump may include a turbo molecular pump, a dry pump, or a combination thereof. 
     An exhaust ring  50  (an exhaust ring assembly) is arranged around the substrate support  11 . The exhaust ring  50  is an annular member and is provided between the sidewall of the plasma processing chamber  10  and the sidewall of the substrate support  11 . The exhaust ring  50  separates the interior of the plasma processing chamber  10  into the plasma processing space  10   s  and an exhaust space  10   t . The exhaust ring  50  has a double structure in which two plate-shaped members are overlapped with each other. 
     [Exhaust Ring] 
     In the exhaust ring having the double structure, there is a trade-off relationship in that, when an opening ratio due to the exhaust holes is high, the exhaust efficiency is improved, but plasma is likely to leak from the plasma processing space  10   s  to the exhaust space  10   t , and when the opening ratio is low, plasma is less likely to leak, but the exhaust efficiency is reduced. That is, in order to efficiently exhaust the exhaust gas from the plasma processing space  10   s , there is a method of enlarging the exhaust hole of the exhaust ring  50  to increase the conductance, but this leads to the leakage of plasma from the exhaust ring  50 . On the other hand, in the exhaust ring  50  in which the exhaust hole is made small so that plasma does not leak and the effect of confining plasma is strengthened, the flow of the exhaust gas is obstructed, which causes a decrease in the exhaust efficiency. In consideration of such a trade-off, the present embodiment proposes a shape of the exhaust ring  50  that achieves both improvement of exhaust efficiency and suppression of plasma leakage. 
     The shape of the exhaust ring  50  according to the present embodiment will be described with reference to  FIGS. 3A to 6C .  FIGS. 3A to 3C  are views illustrating examples of respective members of the exhaust ring  50  having a double structure according to an embodiment.  FIGS. 4A and 4B  are views illustrating examples of a parallel type exhaust ring  50  and a cylindrical type exhaust ring  50  according to an embodiment.  FIGS. 5A and 5B  are a top view and a cross-sectional view of a portion of the exhaust ring  50  according to an embodiment.  FIGS. 6A to 6C  are views illustrating examples of flows of gas passing through exhaust rings  50  according to an embodiment and a comparative example. 
     As illustrated in  FIGS. 3A to 3C , the exhaust ring  50  includes a first member  50 U (first annular member) of an annular shape having a plurality of exhaust holes and a second member  50 D (second annular member) of an annular shape having a plurality of exhaust holes. The first member  50 U and the second member  50 D are arranged to overlap each other.  FIG. 3A  is a top view of the first member  50 U, and  FIG. 3B  is a top view of the second member  50 D.  FIG. 3C  is a view obtained when the exhaust ring  50  having a double structure in which the first member  50 U and the second member  50 D are overlapped with each other is viewed from above. The exhaust ring  50  is arranged horizontally around the substrate support  11  as illustrated in  FIG. 2  in the state in which the radial directions of the first member  50 U and the second member  50 D are horizontally overlapped with each other to form a double structure. That is, the exhaust rings  50  illustrated in  FIG. 2  and  FIGS. 3A to 3C  are parallel type exhaust rings. 
     The first member  50 U of  FIG. 3A  includes circular inner and outer frames  50   s   1  and  50   t   1 , which are concentrically arranged, and a plurality of rod-shaped base materials  50   a  (first rod-shaped portions), which are radically bridged between the circular inner and outer frames  50   s   1  and  50   t   1  over 360 degrees. The plurality of base materials  50   a  are arranged at equal pitches between the inner and outer frames  50   s   1  and  50   t   1 , whereby a plurality of slits are formed radially between the inner and outer frames  50   s   1  and  50   t   1 . The plurality of radially-formed slits function as exhaust holes H 1  (first exhaust holes) for exhausting gas. 
     Referring to an enlarged view (lower figure) of a region V of the first member  50 U in  FIG. 3A , the top surfaces of the base materials  50   a  including the base materials  50   a   1 ,  50   a   2 ,  50   a   3  . . . are chamfered. For example, opposite sides of the top surface of each base material  50   a   2  are chamfered, and inclined surfaces a and b are formed on the opposite sides of the top surface. That is, the top surface of each base material  50   a  has a tapered shape. In the example of  FIG. 3A , a surface e of the center of the top surface of each base material  50   a  is flat, but the flat surface e may be omitted. When there is the flat surface e, as illustrated in the right figure of  FIG. 4A  and  FIG. 5A , each base material  50   a  has a shape in which a flat surface is present in the center of the top surface and inclined surfaces are present on opposite sides of the same. When there is no flat surface e, as illustrated in  FIG. 5B , each base material  50   a  has a shape in which there is no flat surface in the center of the top surface and an apex is formed in the center by inclined surfaces. 
     The second member  50 D of  FIG. 3B  includes circular inner and outer frames  50   s   2  and  50   t   2 , which are concentrically arranged, and a plurality of rod-shaped base materials  50   b  (second rod-shaped portions), which are radically bridged between the circular inner and outer frames  50   s   2  and  50   t   2  over 360 degrees. The plurality of base materials  50   b  are arranged at equal pitches between the inner and outer frames  50   s   2  and  50   t   2 , whereby a plurality of slits are formed radially between the inner and other frames  50   s   2  and  50   t   2 . The plurality of radially-formed slits function as exhaust holes H 2  (second exhaust holes) for exhausting gas. 
     Referring to an enlarged view (lower figure) of a region V of  FIG. 3B  having the same arrangement as the region V of  FIG. 3A , the top surfaces of the base materials  50   b  including the base materials  50   b   1 ,  50   b   2 ,  50   b   3  . . . are chamfered. For example, opposite sides of the top surface of each base material  50   b   2  are chamfered, and inclined surfaces c and d are formed on the opposite sides of the top surface. That is, the top surface of each base material  50   b  has a tapered shape. In the example of  FIG. 3B , a surface f of the center of the top surface of each base material  50   a  is flat, but the flat surface f may be omitted. When there is the flat surface f, as illustrated in the right figure of  FIG. 4A , each base material  50   b  has a shape in which a flat surface is present in the center of the top surface and inclined surfaces are present on opposite sides of the same. When there is no flat surface f, as illustrated in  FIGS. 5A and 5B , each base material  50   b  has a shape in which there is no flat surface in the center of the top surface and an apex is formed in the center by inclined surfaces. 
     Top surfaces of portions other than the exhaust holes of the first member  50 U and top surfaces of portions other than the exhaust holes of the second member  50 D become end surfaces on the upstream side through which gas flows. This makes it possible to improve the flow of the exhaust gas. The portions of the first member  50 U other than the exhaust holes H 1  are the portions of the base materials  50   a , and the portions of the second member  50 D other than the exhaust holes H 2  are the portions of the base materials  50   b.    
     The inner frame  50   s   1  and the inner frame  50   s   2  have the same diameter, and the outer frame  50   t   1  and the outer frame  50   t   2  have the same diameter. Therefore, in the state of  FIG. 3C  in which the first member  50 U is stacked on the second member  50 D, the inner frame  50   s   2  and the outer frame  50   t   2  are arranged under the inner frame  50   s   1  and the outer frame  50   t   1  and is invisible in a top view. 
     In the exhaust ring  50  according to the present embodiment, the first member  50 U is stacked on the second member  50 D. As illustrated in the B-B cross section of  FIG. 4A , no clearance (gap) is provided between the first member  50 U and the second member  50 D. Even if no gap is provided between the first member  50 U and the second member  50 D, the base materials  50   a  and the base materials  50   b  do not come into contact with each other so that exhaust passages are formed. However, a gap may be provided between the first member  50 U and the second member  50 D. The exhaust ring  50  has a shape in which the first member  50 U and the second member  50 D are integrated with each other, and may have a configuration in which slits are formed between the base materials  50   a  and the base materials  50   b  as illustrated in  FIGS. 4A and 4B . 
     As illustrated in  FIGS. 5A and 5B , the horizontal width of the plurality of base materials  50   a  of the first member  50 U is the same as the width of the slits (exhaust holes H 2 ) between the plurality of base materials  50   b  of the second member  50 D. The horizontal width of the plurality of base materials  50   b  of the second member  50 D is the same as the width of the slits (exhaust holes H 1 ) between the plurality of base materials  50   a  of the first member  50 U. In this way, the phases of the slits of the first member  50 U and the slits of the second member  50 D are shifted. As a result, in the state in which the first member  50 U is stacked on the second member  50 D, as partially illustrated in  FIGS. 5A and 5B , the base materials  50   b  having, in a top view, the same width and length as the width and length of the slits (exhaust holes H 1 ) formed in the first member  50 U are arranged below the slits. In addition, the base materials  50   a  having, in a top view, the same width and length as the width and length of the slits (exhaust holes H 2 ) are arranged on the slits formed in the second member  50 D. 
     As a result, as illustrated in the enlarged view of the region V of  FIG. 3C  having the same arrangement as the region V of  FIGS. 3A and 3B  and  FIGS. 5A and 5B , the exhaust space  10   t  is invisible from the plasma processing space  10   s  side in a top view. 
     In addition, the exhaust holes H in the first member  50 U and the exhaust holes H in the second member  50 D do not overlap each other when viewed from the top surface side, that is, in the direction in which the gas flows. In addition, the portions of the first member  50 U other than the exhaust holes H 1  and the portions of the second member  50 D other than the exhaust holes H 2  do not overlap each other when viewed in the flow direction of the exhaust gas. This will be described with reference to  FIGS. 5A and 5B . 
     The upper figure of  FIG. 5A  is a view schematically illustrating the region V 2  of the enlarged view (the lower figure) of  FIG. 3C  in the state of being further enlarged. The lower figure of  FIG. 5A  is a view illustrating a cross section taken along line I-I in the upper figure of  FIG. 5A . As indicated by the arrows in the lower figure of  FIG. 5A , the exhaust gas flows from the top to the bottom of the drawing sheet surface. That is, the gas flows from above the base materials  50   a  of the first member  50 U to the downstream side by passing through the exhaust holes H 1  between the base materials  50   a  and passing through the exhaust holes H 2  between the base materials  50   b  of the second member  50 D. When the exhaust ring  50  is arranged in the plasma processing apparatus  1 , the upstream side is the plasma processing space  10   s , and the downstream side is the exhaust space  10   t.    
     As illustrated in the lower figure of  FIG. 5A , the exhaust holes H 1  between the base materials  50   a  of the first member  50 U and the exhaust holes H 2  between the base materials  50   b  of the second member  50 D do not overlap each other when viewed in the exhaust gas flow direction. However, due to machining, some overlap is allowed. In addition, the portions of the first member  50 U other than the exhaust holes H 1  and the portions of the second member  50 D other than the exhaust holes H 2  do not overlap each other when viewed in the exhaust gas flow direction. However, due to machining, some overlap is allowed. 
     Furthermore, at least the upstream-side end surfaces a, b, c, and d of the portions of the first member  50 U other than the exhaust holes H 1  and the portion of the second member  50 D other than the exhaust holes H 2  have a tapered shape. The tapered end surfaces a, b, c, and d are formed at an angle of 45 degrees with respect to the exhaust gas flow direction in the vertical direction. However, without being limited thereto, at least the upstream-side tapered end surfaces a, b, c, and d may be formed at an angle of 45 degrees or more with respect to the exhaust gas flow direction in the vertical direction. 
     The sizes of the base materials  50   a  and the base materials  50   b  may be varied. The following changes may also be made to the shapes of the base materials  50   a  and the base materials  50   b . For example, in the example of  FIG. 5B , the cross-sectional shapes of the base materials  50   a  and the base materials  50   b  are rhombuses having the same shape. In contrast, in the example of  FIG. 5A , the cross-sectional shapes of the base materials  50   a  and the base materials  50   b  are different from each other. The cross-sectional shape of the base materials  50   b  is a rhombus, but the cross-sectional shape of the base materials  50   a  is a hexagon. That is, as illustrated in  FIG. 5A , the vertices of the base materials  50   a  on the upstream side in the direction in which the exhaust gas flows and the opposing surfaces may be formed flat. 
     In addition, the left and right vertices of the base materials  50   a  and the base materials  50   b  may be formed flat. In order to improve the gas flow, it may be better not to flatten the vertices (angles formed by the end surfaces c and d) of the base materials  50   b  on the downstream side in the gas flow direction. When the vertices of the base materials  50   b  on the downstream side in the gas flow direction are formed at an angle of 90 degrees or less by the end surfaces c and d, the shapes of the base materials  50   b  may be a rhombus, a hexagon, or a triangle. Other shapes may be used. When the vertices of the base materials  50   a  on the upstream side in the gas flow direction are formed at an angle of 90 degrees or less by the end surfaces a and b, the shape of the base materials  50   a  may be a rhombus, a hexagon, or a triangle. 
     In the exhaust rings  150  according to a comparative example illustrated in  FIGS. 6A and 6B , the cross sections of the members  150   a  and the members  150   b  are rectangular, and the end surfaces of these members do not have a tapered shape. In this case, in both of  FIG. 6A  in which the members  150   a  and the members  150   b  overlap when viewed in the direction in which the gas flows on the drawing sheet surface and  FIG. 6B  in which the members  150   a  and the members  150   b  do not overlap when viewed in the direction in which the gas flows on the drawing sheet surface, the folding angle of the exhaust gas when flowing from the base materials  50   a  to the base materials  50   b  becomes about 90 degrees. In addition, the folding angle of the exhaust gas when flowing from the base materials  50   b  to the downstream side of the base materials  50   b  becomes about 90 degrees, and thus the total folding angle of the exhaust gas passing through the exhaust ring  50  becomes approximately 180 degrees. 
     In contrast, according to the exhaust ring  50  including the base materials  50   a  and  50   b  having the shapes and arrangements according to the present embodiment, as illustrated in  FIGS. 5A and 5B , when the cross sections of the base materials  50   a  and  50   b  are viewed, the angle is formed at 45 degrees with respect to the vertical direction of the gas flow. As a result, as illustrated in  FIG. 6C , the folding angle of the exhaust gas when flowing from the base materials  50   a  to the base materials  50   b  becomes about 45 degrees, and the folding angle of the exhaust gas when flowing from the base materials  50   b  to the downstream side of the base materials  50   b  becomes about 45 degrees. Therefore, the total folding angle of the exhaust gas passing through the exhaust ring  50  is approximately 90 degrees. As a result, with the exhaust ring  50  according to the present embodiment, the conductance of the exhaust gas becomes higher than that in the comparative example, and the exhaust efficiency can be improved. 
       FIGS. 7A and 7B  are views illustrating examples of flows of gas passing through exhaust rings in another comparative example.  FIG. 7A  illustrates an example of an exhaust ring  150  provided with a chevron (mountain-shaped) slit as an exhaust hole.  FIG. 7B  is an example of an exhaust ring  150  provided with an oblique slit as an exhaust hole. In these cases, opposite surfaces can be shielded with one slit. Compared to these exhaust rings  150 , in the exhaust ring  50  according to the present embodiment, as illustrated in the base materials  50   a  and the base materials  50   b  of  FIG. 6C , since the volume fraction of the exhaust holes H 1  and the exhaust holes H 2  in the cross section of the exhaust ring  50  is high, the exhaust efficiency is higher than that in the comparative example. In addition, in the slit shapes of  FIGS. 7A and 7B , it is difficult to cover the entire surfaces of the exhaust hole with thermally-sprayed films through thermal spraying to be described later. 
     As described above, with the exhaust ring  50  according to the present embodiment, since the volume fraction of the exhaust holes H in the cross section of the exhaust ring  50  is high, the exhaust efficiency can be improved. In addition, the exhaust ring  50  of the present embodiment has a structure in which the exhaust space  10   t  is not visible from the side of the plasma processing space  10   s . This makes it possible to suppress leakage of plasma. Therefore, with the exhaust ring  50  of the present embodiment, it is possible to achieve both improvement of exhaust efficiency and suppression of plasma leakage. 
     In the above, the parallel type exhaust ring  50  in which the first member  50 U and the second member  50 D are arranged to be overlapped with each other without a gap has been described. However, the first member  50 U and the second member  50 D may be arranged to be overlapped with each other while being spaced apart from each other. 
     As illustrated in  FIG. 4B , a cylindrical exhaust ring  50  may be used. In this case, the first member  50 U and the second member  50 D are arranged to be overlapped with each other in the vertical direction. In this case, the first member  50 U is arranged inside and the second member  50 D is arranged outside. Exhaust gas is exhausted from the inside to the outside of the cylindrical exhaust ring  50 . That is, the exhaust gas flow direction is oriented radially outward from the inside of the exhaust ring  50 . 
     Referring to the C-C cross section of  FIG. 4B , the first member  50 U and the second member  50 D are also arranged to be overlapped with each other in the cylindrical exhaust ring  50  without any spacing therebetween. However, the first member  50 U and the second member  50 D may be arranged to be overlapped with each other with a gap therebetween. 
     In any type of exhaust ring  50 , the first member  50 U and the second member  50 D may be arranged to be overlapped with each other at a distance of at least half the thickness of the second member  50 D in the exhaust gas flow direction. That is, the vertices of the base materials  50   b  of the second member  50 D oriented toward the upstream side with respect to the exhaust gas flow direction may overlap the first member  50 U in the exhaust gas flow direction when the overlapping is not in excess of half the thickness of the base materials  50   b . When the second member  50 D overlaps the first member  50 U in excess of half the thickness of the base materials  50   b  in the exhaust gas flow direction, the exhaust efficiency may be deteriorated. 
     In addition, the first member  50 U and the second member  50 D may be formed integrally with each other. In the parallel type exhaust ring  50  illustrated in  FIG. 4A , the first member  50 U and the second member  50 D may also be formed integrally with each other. 
     In the above, the exhaust ring  50 , which can achieve both improvement of exhaust efficiency and suppression of plasma leakage by preventing deterioration of exhaust efficiency when the exhaust ring  50  has a double structure by optimizing the shape, has been described. 
     [Thermal Spraying] 
     Next, with reference to  FIGS. 8A to 8C  and  FIGS. 9A to 9C , an example of a method of thermal spraying an exhaust ring  50  according to an embodiment will be described in comparison with an exhaust ring  150  of a comparative example.  FIGS. 8A to 8C  and  FIGS. 9A to 9C  are views illustrating examples of a method of thermal spraying the exhaust ring  50  according to an embodiment in comparison with the exhaust ring  150  of the comparative example. The exhaust ring  50  has a plurality of exhaust holes H configured to allow communication between the plasma processing space  10   s  and the exhaust space  10   t  in order to exhaust the exhaust gas from the plasma processing space  10   s  to the exhaust space  10   t . The surface of the exhaust ring  50  is exposed to plasma, including the inner portions of the exhaust holes H. Therefore, a thermally-sprayed film is formed on the surfaces of the first member  50 U and the second member  50 D. The surfaces of the first member  50 U and the second member  50 D are preferably coated with a plasma-resistant thermally-sprayed film such as Y 2 O 3  or the like. 
     However, as illustrated in  FIG. 8A , in a rectangular exhaust ring  150  of the comparative example, a position P 1  in which particles of the sprayed thermally-spraying material do not reach the inner portion of an exhaust hole is generated depending on an aspect ratio of the exhaust hole. In the example of  FIG. 8A , the angle of the thermally-spraying material to be sprayed with respect to the sidewall of the exhaust hole H is less than 45 degrees, and the thermally-spraying material does not reach the position P 1  due to the narrow exhaust hole. Thus, the entire surface of the exhaust hole H cannot be coated with the thermally-sprayed film. 
     In the example of  FIG. 9A , during thermal spraying, the spray may not reach the portion in which the first member  150 U and the second member  150 D overlap (for example, P 3 ). As a countermeasure, for example, changing the spraying angle of the thermally-spraying material may be considered, but there is a concern that the thickness of the thermally-sprayed film will be uneven. As illustrated in  FIG. 8B , depending on the shape of the exhaust ring  50 , the thermally-spraying material cannot be sprayed at an angle. Thus, the inner portions of the exhaust holes H may not be coated with the thermally-sprayed film. In the example of  FIG. 8B , when the thermally-spraying material is sprayed while moving in the x direction, the inner portion of the corner of the exhaust ring  50  cannot be subjected to thermal spraying. 
     In the example of  FIG. 9B , the thermally-spraying material is sprayed to the first member  150 U and the second member  150 D which have a large aspect ratio E of exhaust holes and are arranged at intervals to secure a gas flow path F. In this case, in a first round of thermal spraying, the side surface of the exhaust hole H and the top surface of the first member  150 U are subjected to thermal spraying. Subsequently, in a second round of thermal spraying, the bottom surface of the exhaust hole H (the top surface of the second member  150 D) and the top surface of the first member  150 U are subjected to thermal spraying. Then, on the top surface of the first member  150 U which has been subjected to thermal spraying twice, since the number of times of thermal spraying is larger than that of other portions, a difference is caused in the thickness of a thermally-sprayed film  51 . As a result, the thermally-sprayed film  51  cannot be formed uniformly. 
     In contrast, in the exhaust ring  50  according to the present embodiment, the tapered end surfaces forming the exhaust hole H are formed at an angle θ of 45 degrees or more with respect to the vertical direction of the exhaust gas flow. Thus, the entire inner portion of the exhaust hole H can be coated with the thermally-sprayed film. Therefore, as illustrated in  FIG. 9C , even when the thermally-spraying material is sprayed perpendicularly to the exhaust ring  50 , it is possible to obtain a thermal spraying angle θ of 45 degrees or more with respect to the interior of the exhaust hole H 1  or H 2 . Therefore, a thermally-sprayed film can be formed on the entire exhaust hole H. That is, with the exhaust ring  50  according to the present embodiment, the thermally-spraying material may be sprayed vertically while moving the thermally-spraying material horizontally with respect to the exhaust ring  50 . In the exhaust ring  50  according to the present embodiment, the interior of the exhaust hole H can be coated with a single round of thermal spraying. As a result, the film thickness can be made uniform. 
     As described above, with the exhaust ring  50  according to the present embodiment, the entire shape and the shape of the inlets in the direction in which the exhaust gas flows are optimized. As a result, while maintaining a high volume fraction of the exhaust holes H in the cross sections of the base materials  50   a  and the base materials  50   b , there is no gap in the exhaust ring  50  when viewed from the side of the plasma processing space  10   s , and thus plasma leakage can be suppressed. Furthermore, since the exhaust gas can be exhausted when the folding angle of the exhaust gas flow in the exhaust ring  50  is less than 90 degrees, the exhaust efficiency can be improved. In this way, it is possible to provide the exhaust ring  50  that achieves both improvement in exhaust efficiency and suppression of plasma leakage. In addition, according to the shape of the exhaust ring  50 , the surface of the exhaust ring  50  can be uniformly coated with the thermally-sprayed film. 
     The plasma processing apparatus  1  of the present disclosure is applicable to any type of apparatus of an atomic layer deposition (ALD) apparatus, a capacitively coupled plasma (CCP) apparatus, an inductively coupled plasma (ICP) apparatus, a radial line slot antenna (RLSA) apparatus, an electron cyclotron resonance plasma (ECRP) apparatus, or a helicon wave plasma (HWP) apparatus. 
     According to an aspect, it is possible to provide an exhaust ring capable of achieving both improvement of exhaust efficiency and suppression of plasma leakage. 
     It should be understood that the exhaust rings and the plasma processing apparatus according to the embodiments disclosed herein are exemplary in all respects and are not restrictive. The embodiments can be modified and improved in various forms without departing from the scope and spirit of the appended claims. The matters described in the aforementioned embodiments may have other configurations to the extent that they are not inconsistent, and may be combined to the extent that they are not inconsistent.