Patent Publication Number: US-2012031562-A1

Title: Plasma processing apparatus

Description:
TECHNICAL FIELD 
     The present invention relates to an inductively coupled plasma processing device that can be used for the surface processing of a base body or for other purposes. 
     BACKGROUND ART 
     Inductively coupled plasma processing devices are widely used for the thin-film formation on or the etching process of the surface of a base body. In inductively coupled plasma processing devices, a plasma production gas, such as hydrogen gas, is introduced into a vacuum container, after which a radio-frequency electric field is induced to decompose the plasma production gas and thereby produce plasma. Subsequently, another kind of gas, which serves as a film-forming material gas or an etching gas, is introduced into the vacuum container. In the former case, the molecules of the film-forming material gas are decomposed by the plasma and deposited on a base body. In the latter case, the molecules of the etching gas are decomposed into ions or radicals for the etching process. 
     Patent Document 1 discloses a plasma processing device using an external antenna, in which a radio-frequency antenna for inducing a radio-frequency electric field is disposed above the ceiling of the vacuum container and the portion of the ceiling located directly below the radio-frequency antenna is made of a dielectric material serving as a window for allowing the passage of the induced radio-frequency electric field. In this external antenna type plasma processing device, when the device size is increased to deal with the recent increase in the size of the base body to be processed, it is necessary to increase the thickness of the dielectric window in order to maintain its mechanical strength, which results in a decrease in the strength of the radio-frequency electric field introduced into the vacuum container. Given this problem, an internal antenna type plasma processing device, in which the radio-frequency antenna is provided inside the vacuum container, has also been conventionally used (see Patent Documents 2 and 3). 
     The invention described in Patent Document 3 uses a radio-frequency antenna consisting of a one-dimensional conductor that is terminated without completing one turn (which corresponds to an inductively coupled antenna with the number of turns less than one), such as a U-shaped or semicircular antenna. Such a radio-frequency antenna has an inductance lower than that of an inductively coupled antenna whose number of turns is equal to or greater than one. The lower inductance reduces the radio-frequency voltage occurring at both ends of the radio-frequency antenna and thereby suppresses radio-frequency fluctuation of the plasma potential due to electrostatic coupling to the generated plasma. As a result, an excessive loss of electrons to the ground potential due to the fluctuation of the plasma potential is decreased, whereby the plasma potential is decreased. Therefore, a film formation process with a low level of ion damage to the base body can be performed. 
     BACKGROUND ART DOCUMENT 
     Patent Document 
     
         
         Patent Document 1: JP-A 8-227878 (Paragraph [0010] and FIG. 5) 
         Patent Document 2: JP-A 11-317299 (Paragraphs [0044]-[0046] and FIGS. 1-2) 
         Patent Document 3: JP-A 2001-035697 (Paragraphs [0050]-[0051] and FIG. 11) 
       
    
     DISCLOSURE OF THE INVENTION 
     Problem to be Solved by the Invention 
     In the internal antenna type plasma processing device, the ions in the plasma are accelerated toward the radio-frequency antenna by a self-bias DC voltage which occurs between the conductor of the radio-frequency antenna and the plasma. Therefore, the conductor of the radio-frequency antenna itself undergoes sputtering, which shortens the life of the conductor. Furthermore, the atoms or ions sputtered from the conductor are mixed in the plasma and adhere to the surface of the base body being processed or the inner wall of the vacuum container, causing impurities to be mixed in the thin film being formed or the base body being etched. Another problem of the internal antenna type is that the temperature of the radio-frequency antenna conductor increases since the radio-frequency antenna conductor is located within the plasma. A change in the temperature of the radio-frequency antenna changes the impedance of the radio-frequency antenna, which prevents stable supply of power to the plasma. To address these problems, in the invention described in Patent Document 2, the radio-frequency antenna is sheathed in a pipe made of a dielectric (insulating) material, such as ceramic or quartz, which is less likely to be sputtered than the material of the radio-frequency antenna conductor, such as copper or aluminum, and cooling water is passed through this dielectric pipe. However, this configuration requires both an electrical connector for inputting a radio-frequency power and a connector for supplying or discharging the cooling water to be provided at the ends of the antenna conductor and the dielectric pipe. Such a structure will be complex, making it difficult to attach or detach the antenna or perform maintenance and inspection thereof. 
     In the internal antenna type, since the radio-frequency antenna protrudes into the internal space of the vacuum container, the plasma is produced in the vicinity of the radio-frequency antenna. Therefore, the plasma density particularly increases in the vicinity of the radio-frequency antenna and the density distribution becomes less uniform. Furthermore, since the radio-frequency antenna is located within the vacuum container, the material of the thin film used in the film formation process or a by-product resulting from the etching process may possibly adhere to the surface of the radio-frequency antenna (or a dielectric pipe around this antenna). Such a material or by-product may fall onto the surface of the base body and form so-called particles. 
     Furthermore, as compared to the external antenna type, the internal antenna type needs a vacuum container having a larger capacity in order to ensure a space for the radio-frequency antenna within the vacuum container. Therefore, the gas or plasma easily diffuses, which decreases the amount of ions or radicals reaching the base body and lowers the film-formation rate or etching rate. 
     The problem to be solved by the present invention is to provide a plasma processing device capable of inducing a strong radio-frequency electric field within a vacuum container while preventing sputtering of the antenna conductor, an increase in the temperature of the antenna conductor and the formation of particles. 
     Means for Solving the Problems 
     A plasma processing device according to the present invention aimed at solving the aforementioned problem includes: 
     a) a vacuum container; 
     b) an antenna-placing section provided between an inner surface and an outer surface of a wall of the vacuum container; 
     c) a radio-frequency antenna placed in the antenna-placing section; and 
     d) a dielectric separating member for separating the antenna-placing section from an internal space of the vacuum container. 
     In the plasma processing device according to the present invention, the radio-frequency antenna is placed in the antenna-placing section provided between the inner and outer surfaces of a wall of the vacuum container. Therefore, a stronger radio-frequency electric field can be induced within the vacuum container as compared to the external antenna type. 
     Since the radio-frequency antenna is separated from the internal space of the vacuum container by a dielectric separating member, the formation of particles and the sputtering of the radio-frequency antenna are prevented. Simultaneously, an increase in the temperature of the radio-frequency antenna is suppressed. 
     Since it is unnecessary to provide a space for placing the radio-frequency antenna within the vacuum container, the capacity of the vacuum container can be smaller than in the case of the internal antenna type. Therefore, the diffusion of the gas or plasma is suppressed, which increases the amount of ions or radicals reaching the base body and improves the film-formation rate or the etching rate. 
     The separating member may be a dielectric member provided apart from the wall of the vacuum container. Alternatively, if the wall of the vacuum container is made of a dielectric material, a portion of the wall may be used as the separating member. 
     Although the radio-frequency antenna may be embedded in the wall, it is easier to place it in a hollow space formed between the aforementioned inner and outer surfaces. In the former case, the portion of the wall of the vacuum container in which the radio-frequency antenna is embedded corresponds to the antenna-placing section. In the latter case, the hollow space corresponds to the antenna-placing section. 
     The hollow space may be a hermetically closed space. This design prevents foreign matters from entering the hollow space. When this hollow space is in the vacuum state or filled with an inert gas, no unnecessary electric discharge occurs in the hollow space. 
     The hollow space may be filled with a solid dielectric material. This also prevents the occurrence of unnecessary electric discharge in the hollow space. In this case, it is unnecessary to hermetically close the hollow space. Instead of using the hollow space, it is possible to adopt the structure in which at least a portion of the wall is made of a solid dielectric and the radio-frequency antenna is embedded in the solid dielectric. 
     A cover may be provided on the outer-surface side of the hollow space. The use of such a cover facilitates maintenance, inspection or similar tasks; when the cover is opened, the radio-frequency antenna can be easily removed from the hollow space through the wall of the vacuum container to the outside and then set to the original position. Furthermore, the radio-frequency antenna may be fixed to the cover. In this case, users can more easily remove or set the radio-frequency antenna by merely detaching or attaching the cover. 
     The plasma processing device according to the present invention may be provided with a plurality of antenna-placing sections. This design further improves the uniformity in the density of the plasma created within the vacuum container. 
     Effect of the Invention 
     The plasma processing device according to the present invention is capable of inducing a strong radio-frequency electric field within a vacuum container while preventing sputtering of the antenna conductor, an increase in the temperature of the antenna conductor and the formation of particles. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a vertical sectional view of the first embodiment of the plasma processing device according to the present invention and  FIG. 1B  is a vertical sectional view of a radio-frequency antenna unit  20  used in this plasma processing device. 
         FIGS. 2A-2C  are a perspective view, a top view and a side view showing the shape of a radio-frequency antenna  21  used in the plasma processing device of the present embodiment. 
         FIG. 3  is a top view showing one example of the connection between radio-frequency antennae and radio-frequency power sources. 
         FIG. 4  is an enlarged vertical sectional view showing a first variation of the first embodiment. 
         FIG. 5  is an enlarged vertical sectional view showing a second variation of the first embodiment. 
         FIG. 6  is an enlarged vertical sectional view of the second embodiment of the plasma processing device according to the present invention. 
         FIG. 7  is an enlarged vertical sectional view of the third embodiment of the plasma processing device according to the present invention. 
         FIG. 8  is an enlarged vertical sectional view of the fourth embodiment of the plasma processing device according to the present invention. 
         FIG. 9A  is an enlarged vertical sectional view of the fifth embodiment of the plasma processing device according to the present invention, and  FIG. 9B  is a top view of the radio-frequency antenna  41  used in this embodiment, 
         FIG. 10  is an enlarged vertical sectional view of the sixth embodiment of the plasma processing device according to the present invention. 
         FIG. 11A  is an enlarged vertical sectional view of the seventh embodiment of the plasma processing device according to the present invention, and  FIG. 11B  is a top view showing the construction of a Faraday electrode  51  and surrounding components. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Embodiments of the plasma processing device according to the present invention are hereinafter described by means of  FIGS. 1A-11B . 
     First Embodiment 
       FIG. 1A  is a vertical sectional view of a plasma processing device  10  of the first embodiment. This plasma processing device  10  includes a vacuum container  11 , a base-body holder  12  placed in the internal space  112  of the vacuum container, a gas discharge port  13  and gas introduction ports  14  provided in the side wall of the vacuum container  11 , hollow spaces (antenna-placing sections)  113  provided between the outer surface  111 A and the inner surface  111 B of the top wall  111  of the vacuum container  11 , a separating member (separating plate)  16  for separating the hollow space  113  from the internal space  112  of the vacuum container, and a radio-frequency antenna unit  20  attached to the hollow space  113  from the side of the outer surface  111 A. 
     The separating member  16  is made of a dielectric material. Examples of the available materials include oxides, nitrides, carbides and fluorides. Among these materials, quartz, alumina, zirconia, yttria, silicon nitride or silicon carbide can be suitably used. 
     A step  111 C protruding inwards is formed at the lower end of the inner circumferential surface of the hollow space  113 . The separating plate  16  is fixed to this step  111 C in such a manner that its outer circumferential edge is mounted on the step  111 C. The cover  23  has a projecting portion on its lower surface so that it can fit in the hollow space  113  from the outside of the vacuum container  11 . 
     The gas discharge port  13  is connected to a vacuum pump. By this vacuum pump, the air, steam and other contents in the internal space  112  of the vacuum container are discharged through the gas discharge port  13  to create a high vacuum state. The gas introduction port  14  is used for introducing a plasma production gas (e.g. hydrogen gas) and a film-forming material gas into the internal space  112  of the vacuum container. The base body S to be held on the base-body holder  12  is loaded into the internal space  112  of the vacuum container or unloaded from the same space through a base-body transfer opening  15  formed in the side wall of the vacuum container  11 . The base-body transfer opening  15  is hermetically closed except when the base body is loaded into or unloaded from the vacuum container. 
     The radio-frequency antenna unit  20  is hereinafter described.  FIG. 1B  is a vertical sectional view showing the hollow space  113  and surrounding components, including the radio-frequency antenna unit  20 . The radio-frequency antenna unit  20  consists of a cover  23  and a radio-frequency antenna  21 , the cover  23  being made of a metal (e.g. stainless steel) closing the hollow space  113  from the outside of the vacuum container  11 . 
     The radio-frequency antenna  21  is placed within the hollow space  113 , with both ends fixed to the cover  23  via feedthroughs  24 . Since the radio-frequency antenna  21  is fixed to the cover  23  in this manner, the radio-frequency antenna  21  can be easily detached from or attached to the plasma processing device by detaching or attaching the cover  23 . The radio-frequency antenna  21  consists of an electrically conductive pipe, through which a cooling water or similar coolant can be passed. One end of the radio-frequency antenna  21  is connected to the radio-frequency power source, while the other end is connected to a ground. 
     The shape of the radio-frequency antenna  21  is hereinafter described. As shown in  FIGS. 2A-2C , the radio-frequency antenna  21  includes a first U-shape part  212 A and a second U-shape part  212 B consisting of two U-shaped pipes arranged parallel to the separating member  16 , with each pipe having its two ends directed to those of the other pipe. One end  212 A 1  of the first U-shape part  212 A is connected to one end  212 B 1  of the second U-shape part  212 B by a straight connection part  212 C. The other ends  212 A 2  and  212 B 2  of the first and second U-shape parts  212 A and  212 E are bent upward and connected to the cover  23 . 
     The cover  23  is provided with a hollow-space exhaust port  25  for evacuating the hollow space  113 . The gaps between the radio-frequency antenna  21  and the feedthrough  24 , between the feedthrough  24  and the cover  23 , between the cover and the top wall  111 , and between the separating member  16  and the top wall  111  are hermetically sealed by vacuum seals. The hollow space  113  is maintained in a high vacuum state by the hollow-space exhaust port  25  and the vacuum seals. 
     One example of the connection between the radio-frequency antennae  13  and radio-frequency power sources is hereinafter described by means of  FIG. 3 .  FIG. 3  is a top view of the plasma processing device  10  of the present embodiment. The device of the present embodiment uses a total of eight radio-frequency antennae  21  contained in eight hollow spaces  113 , respectively. These eight radio-frequency antennae  21  are divided into two groups, each group including four antennae, and one radio-frequency power source is connected to each group. A power supply end  211  of each of the radio-frequency antennae  21  is connected to each of the four power supply rods  32  extending from a power supply point  31  in four directions. The aforementioned radio-frequency power source is connected to that point  31 . 
     As one example of the operation of the plasma processing device  10  of the present embodiment, the process of depositing a film-forming material on the base body S is hereinafter described. Initially, a base body S is loaded through the base-body transfer opening  15  into the internal space  112  of the vacuum container and placed onto the base-body holder  12 . After the base-body transfer opening  15  is closed, the vacuum pump is energized, whereby the air, steam and other contents in the internal space  112  of the vacuum container are discharged through the gas discharge port  13 , and the air, steam and other contents in the hollow space  113  are also discharged through the hollow-space exhaust port  25 . Thus, the internal space  112  of the vacuum container and the hollow space  113  are evacuated. Subsequently, a plasma production gas and a film-forming material gas are introduced from the gas introduction port  14 . A radio-frequency power is supplied to each radio-frequency antenna  21 , while a coolant is passed through the pipe of the radio-frequency antenna  21 . By this radio-frequency power supply, a radio-frequency electric field is induced around the radio-frequency antenna  21 . This radio-frequency electric field is introduced through the dielectric separating member  16  into the internal space  112  of the vacuum container and ionizes the plasma production gas, whereby plasma is produced. The film-forming material gas, which has been introduced into the internal space  112  of the vacuum container together with the plasma production gas, is decomposed by the resultant plasma, to be deposited on the base body S. 
     As compared to the external antenna type, the plasma processing device  10  of the present embodiment can create a stronger radio-frequency electric field within the internal space  112  of the vacuum container  11  since the radio-frequency antenna  21  is located in the hollow space  113  provided between the outer surface  111 A and the inner surface  111 B of the top wall  111  of the vacuum container. The separation of the hollow space  113  including the radio-frequency antenna  21  from the internal space  112  of the vacuum container by the separating member  16  has the effects of: preventing plasma produced in the aforementioned space from etching the radio-frequency antenna  21  and shortening its life; preventing the material of the radio-frequency antenna  21  from becoming an impurity to be mixed in the film being formed or the base body being processed; and preventing the formation of particles. Furthermore, since the hollow space  113  in which the radio-frequency antenna  21  is placed is maintained in a high vacuum state, no unnecessary electric discharge occurs in the hollow space  113 . 
     In the present embodiment, a magnetic field created in the first U-shape part  212 A of the radio-frequency antenna  21  by an electric current flowing from one end  212 A 1  to the bottom part of the U-shaped body, and a magnetic field created by an electric current flowing from the bottom part of the U-shaped body to the other end  212 A 2 , have vertical components oscillating in the same phase. Magnetic fields having such vertical components are similarly created in the second U-shape part  212 B. As a result, the magnitude of the vertical component of the magnetic field below the antenna will be greater than in the case of using a single straight radio-frequency antenna. Therefore, as compared to the case of using a single straight radio-frequency antenna, a higher plasma density can be achieved under the same strength of the radio-frequency power and/or the same pressure of the plasma production gas, or the same plasma density can be achieved under a lower strength of the radio-frequency power and/or a lower pressure of the plasma production gas. 
     A first variation of the first embodiment is hereinafter described by means of  FIG. 4 . In the present variation, the top wall  111  has no step  111 C; the separating member  16 A is arranged so that it covers the hollow space  113  on the side facing the internal space  112  of the vacuum container. With this design, the hollow space  113  is expanded toward the internal space  112  of the vacuum container and the radio-frequency antenna  21  can be brought closer to the internal space  112  of the vacuum container. The other structural elements are the same as those of the previously described embodiment. 
     A second variation of the first embodiment is hereinafter described by means of  FIG. 5 . In the present variation, a hollow space  113 A is created by boring a hole from the lower surface of the top wall  111  without completely penetrating through the top wall  111 . Accordingly, a portion of the top wall  111  remains intact above the hollow space  113 A. The radio-frequency antenna  21  is fixed to this remaining portion of the top wall  111  via feedthroughs. The hollow-space exhaust port  25 C is also provided in that portion of the top wall  111 . The structure of the separating member  16 A is the same as that of the first variation. 
     Second Embodiment 
     A plasma processing device of the second embodiment is hereinafter described by means of  FIG. 6 . In the present embodiment, a hollow-space inert-gas introduction port  25 A and a hollow-space gas discharge port  25 B are provided in the cover  23  of the radio-frequency antenna unit  20 A in place of the hollow-space exhaust port  25  in the first embodiment. The hollow space  113  can be filled with an inert gas, such as argon or nitrogen, by introducing the inert gas through the hollow-space inert-gas introduction port  25 A to replace air and steam in the hollow space  113  by the inert gas and discharge the air and steam through the hollow-space gas discharge port  25 B to the outside. As a result, similar to the case of evacuating the hollow space  113 , the occurrence of unnecessary electric discharge is prevented. The other structural elements are the same as those of the first embodiment. 
     Third Embodiment 
     A plasma processing device of the third embodiment is hereinafter described by means of  FIG. 7 . In the present embodiment, the hollow space  113  is filled with a dielectric member  27 . Examples of the materials for the dielectric member  27  include polytetrafluoroethylene (PTFE), polyether ether ketone (PEEK) and other resins as well as alumina, silica and other ceramics. The bottom portion of the dielectric member  27  functions as the separating member. The radio-frequency antenna  21 , which is U-shaped similar to the previous embodiment, is directly fixed to the cover  23  without using feedthroughs. Since the radio-frequency antenna  21  is fixed to the cover  23  in this manner, both the radio-frequency antenna  21  and the dielectric member  27  around this antenna  21  are attached to or detached from the vacuum container  11  when the cover  23  is attached to or detached from this container  11 . Accordingly, it can be said that the radio-frequency antenna  21 , the cover  23  and the dielectric member  27  in the present embodiment form one set of the radio-frequency antenna unit  20 B. 
     In the third embodiment, since the hollow space  113  is filled with the dielectric member  27 , no unnecessary electric discharge occurs in the vicinity of the radio-frequency antenna  21 . 
     In place of the dielectric member  27 , a dielectric powder may be filled into the hollow space  113 . In this case, the hollow space  113  should be hermetically closed so that the powder will not leak from the hollow space  113 . 
     Fourth Embodiment 
     In any of the previous examples, the radio-frequency antenna  21  was provided within the hollow space  113 . However, it is possible to embed the radio-frequency antenna  21  between the outer surface  111 A and the inner surface  111 B without using any hollow space, as shown in  FIG. 8 , where the region denoted by numeral  113 B corresponds to the antenna-placing section. In this case, in order to electrically insulate the radio-frequency antenna  21  from the top wall  111  and to prevent unnecessary electric discharge from occurring in the vicinity of the radio-frequency antenna  21 , a dielectric member should be provided between the radio-frequency antenna  21  and the top wall  111 , or the top wall  111  should be made of a dielectric material. In the latter case, the top wall  111  may be entirely made of the dielectric material. However, for the sake of the cost reduction, it is preferable to use the dielectric material only in the portion of the top wall  111  near the radio-frequency antenna  21 . For the herein mentioned dielectric material, the previously listed materials of the dielectric member  27  are similarly available. The portion of the top wall  111  located between the radio-frequency antenna  21  and the internal space  112  of the vacuum container may be made of a dielectric material so that this portion functions as a separating plate  16 B. 
     Fifth Embodiment 
     One example using a radio-frequency antenna having a shape different from any of the previous embodiments is described by means of  FIGS. 9A and 9B . As shown in  FIG. 9B , the radio-frequency antenna  41  in the present embodiment consists of one electrically conductive pipe spirally wound in a plane parallel to the separating member  16 . The other structural elements are the same as those of the first embodiment. By using the radio-frequency antenna  41  having such a shape, it is possible to create a magnetic field over a larger area than in the case of using a straight or U-shaped radio-frequency antenna. 
     Sixth Embodiment 
     In any of the previous embodiments, there was only one radio-frequency antenna placed in each antenna-placing section (hollow space). However, it is possible to provide two or more radio-frequency antennae in one antenna-placing section. In the example shown by the top view in  FIG. 10 , two radio-frequency antennae  21  described in the first embodiment (first radio-frequency antenna  21 A and second radio-frequency antenna  21 B) are provided in the hollow space  113 . The first and second radio-frequency antennae  21 A and  21 B are arranged so that their first and second U-shaped parts  212 A and  212 B are at the same distance from the separating member  16  and their connection parts  212 C are parallel to each other. 
     Seventh Embodiment 
     A seventh embodiment of the plasma processing device according to the present invention is described by means of  FIGS. 11A and 11B . The plasma processing device of the present embodiment is a variation of the plasma processing device  10  of the first embodiment and further includes a Faraday shield  51  placed on the separating member  16  (between the separating member  16  and the radio-frequency antenna  21 ). The Faraday shield  51  is electrically connected to the metallic top wall  111  and further to a ground via the top wall  111 . The Faraday shield  51  stops a DC electric field produced by a self bias between the conductor of the radio-frequency antenna  21  and the plasma and thereby prevents plasma produced in the internal space  112  from impinging on the separating member  16 , so that the life of the separating member  16  will be increased. A dielectric insulating member  52  is inserted between the Faraday shield  51  and the radio-frequency antenna  21  in order to prevent electric discharge from occurring in the space between the shield and the antenna. 
     In the Faraday shield  51 , an almost entire portion of the lower surface is thermally in contact with the separating member  16 , with both ends thermally connected to the top wall  111 . Therefore, the heat from the separating member  16 , which receives energy from the plasma and becomes hotter, is released through the Faraday shield  51  to the top wall  111 . In this manner, an increase in the temperature of the separating member  16  is suppressed and the degradation of the separating member  16  due to the heat is prevented. To further improve this effect, the Faraday shield  51  may be cooled by a coolant, or a means for suppressing the temperature increase, such as a cooling pipe, may be additionally provided apart from the Faraday shield  51 . 
     Other Embodiments 
     The number of radio-frequency antennae  21 , which was eight in the previous embodiments, can be appropriately determined according to the capacity of the vacuum container or other factors. Using only one radio-frequency antenna  21  may be sufficient for a vacuum container having a rather small capacity. Unlike the previous embodiments, in which the radio-frequency antenna unit  20  was provided in the top wall of the vacuum container, the radio-frequency antenna unit may be provided in a different wall, such as the side wall. 
     EXPLANATION OF NUMERALS 
     
         
           10  . . . . Plasma Processing Device 
           11  . . . . Vacuum Container 
           111  . . . . Top Wall of Vacuum Container 
           111 A . . . . Outer Surface of Top Wall of Vacuum Container 
           111 B . . . . Inner Surface of Top Wall of Vacuum Container 
           111 C . . . . Step Formed on Top Wall of Vacuum Container 
           112  . . . . Internal Space 
           113 ,  113 A . . . . Hollow Space (Antenna-Placing Section) 
           113 B . . . . Antenna-Placing Section 
           12  . . . . Base-Body Holder 
           13  . . . . Gas Discharge Port 
           14  . . . . Gas Introduction Port 
           15  . . . . Base-Body Transfer Opening 
           16 ,  16 A,  1613  . . . . Separating Member (Separating Plate) 
           20 ,  20 A,  20 B . . . . Radio-Frequency Antenna Unit 
           21 ,  41  . . . . Radio-Frequency Antenna 
           211  . . . . Power Supply End 
           21 A . . . . First Radio-Frequency Antenna 
           21 B . . . . Second Radio-Frequency Antenna 
           212 A . . . . First U-Shaped Part 
           212 B . . . . Second U-Shaped Part 
           212 C . . . . Connection Part 
           23  . . . . Cover 
           24  . . . . Feedthrough 
           25 ,  25 C . . . . Hollow-Space Exhaust Port 
           25 A . . . . Hollow-Space Inert-Gas Introduction Port 
           25 B . . . . Hollow-Space Gas Discharge Port 
           27  . . . . Dielectric Member 
           31  . . . . Power Supply Point 
           32  . . . . Power Supply Rod 
           51  . . . . Faraday Shield 
           52  . . . . Insulating Member 
         S . . . . Base Body