Patent Publication Number: US-2012031563-A1

Title: Plasma processing device

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 2-4). 
     The inventions described in Patent Documents 3 and 4 use 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. 
     Furthermore, in an invention described in Patent Document 4, an antenna conductor is made to be shorter than one quarter of the wavelength of the radio-frequency waves. This design prevents the generation of standing waves in the antenna conductor and improves the uniformity of the generated plasma. 
     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) 
     Patent Document 4: JP-A 2004-039719 (Paragraphs [0019]-[0021] and FIG. 3) 
     Non-Patent Document 
     Non-Patent Document 1: Yuichi SETSUHARA and Akinori EBE, “Plasma Technologies for Meters-scale Large-Area Processes”,  Journal of The Surface Finishing Society of Japan , The Surface Finishing Society of Japan, May 2005, Vol. 56, No. 5, pp. 18-25. 
     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, if the antenna conductor is exposed, 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. To address this problem, 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. The reason for using a pipe is to supply cooling water for preventing an increase in the temperature of the antenna conductor. 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. Another problem is that the material of the thin film used in the film formation process or a by-product resulting from the etching process adheres 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 and improving the uniformity in the density distribution of the plasma, while preventing formation of particles or contamination of a base body due to the sputtering of the conductor of the radio-frequency antenna. 
     Means for Solving the Problems 
     A plasma processing device according to the present invention aimed at solving the aforementioned problem is an inductively coupled plasma device using a radio-frequency electric discharge, including: 
     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, the radio-frequency antenna being terminated without completing one turn; and 
     d) a dielectric separating member for separating the antenna-placing section from an internal space of the vacuum container, 
     wherein the radio-frequency antenna has a length equal to or shorter than one quarter of a wavelength of radio-frequency waves. 
     In the plasma processing device according to the present invention, an antenna-placing section is provided between the inner and outer surfaces of the wall of the vacuum container. A radio-frequency antenna located in such an antenna-placing section is capable of inducing a radio-frequency electric field in the vacuum container with a strength comparable to that of the radio-frequency electric field created by an internal antenna type plasma processing device. 
     On the other hand, unlike the conventional internal antenna type, the radio-frequency antenna is separated from the internal space of the vacuum container by a dielectric separating member, whereby the formation of particles and the sputtering of the radio-frequency antenna are prevented. Furthermore, 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. 
     Furthermore, the separation between the radio-frequency antenna and the plasma production space prevents the plasma density from being particularly high in the vicinity of the radio-frequency antenna. Therefore, unlike the internal antenna type, the uniformity in the density distribution will not deteriorate. The use of an antenna having a length equal to or shorter than one quarter of the wavelength of the radio-frequency waves is also effective for preventing the deterioration in the uniformity in the density distribution since no standing waves occur in such an antenna. 
     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 the vacuum container and improving the uniformity in the density distribution of the plasma, while preventing formation of particles or contamination of a base body due to the sputtering of the conductor of the radio-frequency antenna. 
    
    
     
       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 an enlarged view of an antenna-placing section  12  and surrounding components. 
         FIG. 2  is a top view showing one example of the connection between radio-frequency antennae and radio-frequency power sources. 
         FIG. 3  is an enlarged vertical sectional view of the second embodiment of the plasma processing device according to the present invention. 
         FIG. 4  is an enlarged vertical sectional view showing a first variation of the plasma processing device according to the second embodiment. 
         FIG. 5  is an enlarged vertical sectional view showing a second variation of the plasma processing device according to the second embodiment. 
         FIG. 6  is an enlarged vertical sectional view of the third embodiment of the plasma processing device according to the present invention. 
         FIG. 7  is a vertical sectional view showing an example in which the radio-frequency antenna is embedded between the outer surface  111 A and the inner surface  111 B. 
         FIGS. 8A and 8B  are a side view and a top view showing an example in which a plate-shaped member is used as a part of the radio-frequency antenna. 
         FIG. 9  is a graph showing the result of an experiment for investigating a change in the plasma density with changing output of the radio-frequency power. 
     
    
    
     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-9 . 
     First Embodiment 
     Initially, a plasma processing device  10  of the first embodiment is described by means of  FIGS. 1A ,  1 B and  2 . The plasma processing device  10  includes a vacuum container  11 , a plurality of antenna-placing sections  12  each of which is located between the outer surface  111 A and the inner surface  111 B of the top wall  111  of the vacuum container  11 , a radio-frequency antenna  13  placed in each antenna-placing section  12 , a dielectric filler  14  in the form of a mass filling the space around the radio-frequency antenna  13  in the antenna-placing section  12 , a dielectric separating member (separating plate)  15  provided on the side of the antenna-placing section  12  facing the inner surface  111 B of the top wall, a cover  16  for covering the antenna-placing section  12  on the outer surface  111 A of the top wall, a gas discharge port  17  and gas introduction ports  18  provided in the side wall of the vacuum container  11 , and a base-body holder  19  provide in the internal space  112  of the vacuum container. 
     Specifically, the antenna-placing section  12  is constructed as follows. A through-hole is formed in a portion of the top wall  111  of the vacuum container  11 . A plate-shaped support member  121  is attached to the top wall  111 , being fitted in the through-hole. The support member  121  has a hollow space bored from above. This space corresponds to the antenna-placing section  12 . As described previously, the radio-frequency antenna  13  and the dielectric filler  14  are provided in the antenna-placing section  12 . The cover  16  has its edge fixed to the upper side of the support member  121 . 
     The radio-frequency antenna  13  is a U-shaped metal pipe. The length of this pipe is 35 cm. Many commercial radio-frequency power sources operate at a frequency of 13.56 MHz. The propagation wavelength of the radio-frequency waves having this frequency is 22 m in free space. However, according to a propagation model in which the coupling with plasma is considered, their propagation wavelength is estimated to be approximately 13 m (Non-Patent Document 1), Accordingly, the length of the aforementioned metal pipe is shorter than one quarter of the propagation wavelength of the 13.56-MHz waves. Both ends of the radio-frequency antenna  13  are fixed to the cover  16  via feedthroughs  161 . The pipe of the radio-frequency antenna  13  functions as a channel through which a coolant (e.g. water) can be passed to cool the radio-frequency antenna  13  while the plasma processing device  10  is being used. 
     The separating member  15  is used for preventing the inside of the antenna-placing section  12  from being exposed to plasma. The separating member  15  will be exposed to plasma and hence should desirably be made of a material having high plasma resistance, such as quartz. On the other hand, the dielectric filler  14 , which will not be exposed to plasma due to the presence of the separating member  15 , should preferably be made of a material having high workability rather than high plasma resistance. Examples of such high-workability materials include polytetrafluoroethylene (PTFE), polyether ether ketone (PEEK) and other kinds of resin. Naturally, alumina, silica and other kinds of ceramics are also available. 
     Vacuum seals are provided between the top wall  111  and the support member  121 , between the support member  121  and the cover  16 , as well as between the cover  16  and the feedthrough  161 , respectively. On the other hand, although the separating member  15  is attached to the support member  121 , no vacuum seal is provided between the separating member  15  and the support member  121 . 
     The gas discharge port  17  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  17 . The gas introduction port  18  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  19  is loaded into the internal space  112  of the vacuum container or unloaded from the same space through a base-body transfer opening  113  formed in the side wall of the vacuum container  11 . The base-body transfer opening  113  is hermetically closed except when the base body is loaded into or unloaded from the vacuum container. 
     One example of the connection between the radio-frequency antennae  13  and radio-frequency power sources is hereinafter described by means of  FIG. 2 . The device in accordance with the present embodiment uses a total of eight radio-frequency antennae  13  contained in eight antenna-placing sections  12 , respectively. These eight radio-frequency antennae  13  are divided into two groups, each group including four antennae, and one radio-frequency power source is connected to each group. One end (power supply end  131 ) of each of the radio-frequency antennae  13  is connected to each of the four power supply rods  134  extending from a power supply point  133  in four directions, to which point the aforementioned radio-frequency power source is connected. The other end (grounded end  132 ) of each of the radio-frequency antennae  13  is connected to a ground. 
     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  113  into the internal space  112  of the vacuum container and placed onto the base-body holder  19 . After the base-body transfer opening  113  is closed, the air, steam and other contents in the internal space  112  of the vacuum container are discharged through the gas discharge port  17  by using the vacuum pump. Subsequently, a plasma production gas and a film-forming material gas are introduced from the gas introduction port  18 . A radio-frequency power is supplied to each radio-frequency antenna  13 , while a coolant is passed through the pipe of the radio-frequency antenna  13 . By this radio-frequency power supply, a radio-frequency electric field is induced around the radio-frequency antenna  13 . This radio-frequency electric field is introduced through the dielectric separating member  15  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  13  is located in the antenna-placing section  12  provided between the outer surface  111 A and the inner surface  111 B of the top wall  111  of the vacuum container. Furthermore, since the antenna-placing section  12  including the radio-frequency antenna  13  is separated from the internal space  112  of the vacuum container by the separating member  15 , the plasma produced in the aforementioned space is prevented from etching the radio-frequency antenna  13  and shortening its life, or the material of the radio-frequency antenna  13  is prevented from being an impurity and mixed in the film being formed or the base body being processed. The dielectric filler  14  in the antenna-placing section  12  prevents unnecessary electric discharge from occurring in the antenna-placing section  12 . 
     In the first embodiment, the dielectric separating member (separating plate)  15  is provided apart from the dielectric filler  14 . If the dielectric filler  14  is made of a material with high plasma resistance, and if a sufficiently thick layer of the dielectric filler  14  is present between the internal space  112  of the vacuum container and the radio-frequency antenna  13 , it is possible to omit the separating member  15  (provided apart from the dielectric filler  14 ) since the aforementioned layer of the dielectric filler  14  functions as the separating member. 
     The dielectric filler  14  used in the first embodiment is in the form of a mass. Alternatively, a dielectric powder may be used, in which case the antenna-placing section  12  should be hermetically closed so that the powder will not leak from the antenna-placing section  12 . 
     Second Embodiment 
     A plasma processing device of the second embodiment is hereinafter described by means of  FIG. 3 . In the following description, the components identical to those used in the plasma processing device  10  of the first embodiment are denoted in  FIG. 3  by the same numerals as used in  FIGS. 1A and 1B  and will not be specifically described. One feature of the present embodiment is that a hollow space  22  serving as the antenna-placing section is formed between the outer surface  111 A and the inner surface  111 B of the top wall  111  of the vacuum container  11 . 
     A dielectric separating member  25  is provided on the side of the inner surface  111 B of the hollow space  22 , while a cover  26  is provided on the side of the outer surface  111 A. The separating member  25  is attached to the wall in such a manner that it is mounted on a step  111 C protruding inwards from the lower end of the inner circumferential surface of the hollow space  22 . The cover  26  has a projecting portion on its lower surface so that it can fit in the hollow space  22  from the outer side of the vacuum container  11 . Both ends of the radio-frequency antenna  13  are fixed to the cover  26  via the feedthroughs  261 . The radio-frequency antenna  13  can be easily detached from or attached to the plasma processing device by detaching or attaching the cover  26 . 
     The hollow space  22  is hermetically sealed with vacuum seals provided between the cover  26  and the top wall  111  as well as between the separating member  25  and the top wall  111 . The cover  26  is provided with a hollow-space exhaust port  27  so that the hollow space  22  can be evacuated independently of the internal space  112  of the vacuum container. 
     The operation of the plasma processing device of the second embodiment is basically the same as that of the plasma processing device  10  of the first embodiment. An important difference is that the hollow space  22  is evacuated by discharging the gas in the hollow space  22  through the hollow-space exhaust port  27  to the outside before the radio-frequency power is supplied to the radio-frequency antenna  13 . This prevents unnecessary electric discharge from occurring in the hollow space  22 . 
     A first variation of the second embodiment is hereinafter described by means of  FIG. 4 . In the present variation, the device has no step  111 C; the separating member  25 A is arranged so that it covers the hollow space  22  on the side facing the internal space  112  of the vacuum container. With this design, the hollow space  22  is expanded toward the internal space  112  of the vacuum container and the radio-frequency antenna  13  can be brought closer to the internal space  112  of the vacuum container. The other structural elements are the same as those of the second embodiment. 
     A second variation of the second embodiment is hereinafter described by means of  FIG. 5 . In the present variation, a hollow space  22 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  22 A. The radio-frequency antenna  13  is fixed to this remaining portion of the top wall  111  via feedthroughs. The hollow-space exhaust port  27 A is also provided in that portion of the top wall  111 . The other structural elements are the same as those of the first variation of the second embodiment. 
     Third Embodiment 
     The plasma processing device of the third embodiment is hereinafter described by means of  FIG. 6 . In the present embodiment, a hollow-space inert-gas introduction port  37 A and a hollow-space gas discharge port  37 B are provided in the cover  36  in place of the hollow-space exhaust port  27  in the second embodiment. The hollow space  22  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  37 A to replace air and steam in the hollow space  22  by the inert gas and discharge the air and steam through the hollow-space gas discharge port  37 B to the outside. As a result, similar to the case of evacuating the hollow space  22 , the occurrence of unnecessary electric discharge is prevented. The other structural elements are the same as those of the second embodiment. 
     Fourth Embodiment 
     In any of the previously described examples, the dielectric filler  14  or hollow space  22  was located between the outer surface  111 A and the inner surface  111 B of the vacuum container. However, it is possible to embed the radio-frequency antenna  13  between the outer surface  111 A and the inner surface  111 B without using any hollow space, as shown in  FIG. 7 , where the region denoted by numeral  42  corresponds to the antenna-placing section. In this case, in order to electrically insulate the radio-frequency antenna  13  from the top wall  111  and to prevent unnecessary electric discharge from occurring in the vicinity of the radio-frequency antenna  13 , a dielectric member should be provided between the radio-frequency antenna  13  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  11  near the radio-frequency antenna  13 . For the herein mentioned dielectric material, the previously listed materials of the dielectric filler  14  are similarly available. The portion of the top wall  111  located between the radio-frequency antenna  13  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  45 . 
     [Variation Common to Previous Embodiments] 
     In any of the previous embodiments, the rod-shaped (tubular) radio-frequency antenna may be replaced with a plate-shaped radio-frequency antenna or a composite radio-frequency antenna including a rod-shaped member combined with a plate-shaped member. By appropriately selecting a rod-shaped antenna or a plate-shaped antenna or combining the two types of antenna, it is possible to adjust the impedance of the antenna.  FIGS. 8A and 8B  show a radio-frequency antenna including a U-shaped metal pipe with a flat bottom face to which a plate-shaped member  51  is attached. The impedance of this antenna can be adjusted by changing the width and/or thickness of the plate-shaped member  51 . 
     In place of the U-shaped radio-frequency antenna used in the previous embodiments, a radio-frequency antenna having a semicircular shape, arc shape or the like may be used. The semicircular or arc-shaped radio-frequency antenna is an inductively coupled antenna whose number of turns is less than one. The inductance of such an antenna is lower than that of an antenna whose number of turns is equal to or greater than one. Therefore, the voltage which occurs in the antenna when a predetermined radio-frequency power is supplied is low, so that plasma can be efficiently produced. The number of antenna-placing sections, 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 antenna-placing section may be sufficient for a vacuum container having a rather small capacity. Unlike the previous embodiments, in which the antenna-placing section was provided in the top wall of the vacuum container, the antenna-placing section may be provided in a different wall, such as the side wall. 
     [First Experiment] 
     For a plasma processing device according to the second embodiment with only one antenna-placing section, the density of the plasma produced within the vacuum container was measured. A U-shaped copper pipe of 6 mm in outer diameter with a bottom section extending parallel to the top wall  111  over a length of 150 mm was used the radio-frequency antenna  13 . The position of the bottom section of the radio-frequency antenna  13  was adjusted to be level with the inner wall surface of the top wall  111  ( FIG. 4 ). A quartz plate of 6 mm in thickness was used as the separating member  15 A. 
     After the vacuum container  11  was evacuated, a mixture gas of hydrogen and argon was introduced into the vacuum container  11  to a pressure of 1 Pa. Subsequently, a radio-frequency power of 1 kW in output and 13.56 MHz in frequency was supplied to the radio-frequency antenna  13  while cooling water was supplied through the antenna pipe. As a result, the vacuum container  11  became filled with plasma. The plasma density at a point located 20 cm away from the bottom section of the U-shaped radio-frequency antenna  13  was 1.2×10 11 /cm 3 . When the output of the radio-frequency power was varied within the range from 1 kW to 3 kW, the plasma density at the aforementioned point changed in proportion to the output of the radio-frequency power ( FIG. 9 ). 
     [Second Experiment] 
     A copper plate of 150 mm in length and 30 mm in width, being held parallel to the top wall  111 , was attached to the bottom section of the radio-frequency antenna  13  used in the first experiment. As compared to the first experiment, the position of the radio-frequency antenna  13  was shifted upwards so as to level the copper plate with the lower surface of the top wall  111 . Using this radio-frequency antenna, an experiment similar to the first experiment was performed. The plasma density at a point located 20 cm away from the copper plate was 1.4×10 11 /cm 3 , which was higher than in the first experiment with no copper plate used. This is probably because the electric current flowing through the radio-frequency antenna (the copper pipe and the copper plate) increased due to a decrease in the impedance of the antenna caused by attaching the copper plate having a width larger than the diameter of the copper pipe. 
     For comparison, the same experiment was performed with the separating member  15 A removed. The result was that the plasma density at the aforementioned point was 1.6×10 11 /cm 3 . Thus, it was confirmed that the use of the separating member  15 A has only a minor effect on the production of plasma. 
     [Third Experiment] 
     Eight sets of the radio-frequency antennas used in the first experiment were connected as shown in  FIG. 2 , and a radio-frequency power of 2 kW in output and 13.56 kHz in frequency were supplied to each power supply point  133 . The other conditions were the same as in the first and second experiments. For each radio-frequency antenna unit, the plasma density at a point located 20 cm away from the bottom section of the U-shaped radio-frequency antenna  13  was measured. The result confirmed that a cloud of plasma having approximately the same strength was produced by any of the radio-frequency antennas. This result suggests that it is possible to improve the uniformity of the plasma within the vacuum container  11  by using a plurality of radio-frequency antennas (or antenna-placing sections including the radio-frequency antennas) each of which produces plasma having approximately the same strength. 
     EXPLANATION OF NUMERALS 
       10  . . . Plasma Processing Device 
       11  . . . Vacuum Container 
       111 ... Top Wall of Vacuum Container  11   
       111 A . . . Outer Surface of Top Wall  111   
       111 B . . . Inner Surface of Top Wall  111   
       111 C . . . Step of Inner Surface  111 B 
       112  . . . Internal Space 
       113  . . . Base-Body Transfer Opening 
       12 ,  42  . . . Antenna-Placing Section 
       121  . . . Support Member 
       13  . . . Radio-Frequency Antenna 
       131  . . . Power Supply End 
       132  . . . Grounded End 
       133  . . . Power Supply Point 
       134  . . . Power Supply Rod 
       14  . . . Dielectric Filler 
       15 ,  15 A,  25 ,  25 A,  45  . . . Separating Member 
       16 ,  26 ,  36  . . . Cover 
       161 ,  261  . . . Feedthrough 
       17  . . . Gas Discharge Port 
       18  . . . Gas Introduction Port 
       19  . . . Base-Body Holder 
       22 ,  22 A . . . Hollow Space 
       27 ,  27 A . . . Hollow-Space Exhaust Port 
       37 A . . . Hollow-Space Inert-Gas Introduction Port 
       37 B . . . Hollow-Space Gas Discharge Port 
       51  . . . Plate-Shaped Member 
     S . . . Base Body