Patent Publication Number: US-2021193439-A1

Title: Plasma processing apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to Japanese Patent Application No. 2019-231039, filed on Dec. 23, 2019, the entire contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to a plasma processing apparatus. 
     BACKGROUND 
     An inductively coupled plasma (ICP) type plasma processing apparatus is known as an example of a plasma processing apparatus. The ICP type plasma processing apparatus employs, for example, a technique for exciting a processing gas by generating an induced electric field in a processing chamber using a coil-shaped outer antenna for supplying a radio frequency power and an inductively coupled coil-shaped inner antenna that is concentric with the outer antenna (see, e.g., Japanese Patent Application Publication No. 2019-067503). 
     SUMMARY 
     The present disclosure provides a plasma processing apparatus capable of improving controllability of gas distribution on a substrate. 
     In accordance with an aspect of the present disclosure, there is provided a plasma processing apparatus including: a chamber including a sidewall and a ceiling plate having a central opening, the sidewall and the ceiling plate defining a plasma processing space; an antenna assembly disposed above the ceiling plate, the antenna assembly including a central region, a first peripheral region and a second peripheral region, the first peripheral region surrounding the central region, and the second peripheral region surrounding the first peripheral region, the central region and the first peripheral region vertically overlapping the central opening; a primary coil disposed in the second peripheral region; a radio frequency (RF) power supply configured to supply an RF signal to the primary coil; and a gas shower disposed in the central opening, the gas shower having a bottom portion exposed to the plasma processing space, the bottom portion having bottom gas injection holes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The objects and features of the present disclosure will become apparent from the following description of embodiments, given in conjunction with the accompanying drawings, in which: 
         FIG. 1  shows an example of a plasma processing system according to a first embodiment of the present disclosure; 
         FIG. 2  is a schematic perspective view showing an example of an antenna according to the first embodiment; 
         FIG. 3  shows an example of an arrangement of an inner coil and an outer coil according to the first embodiment; 
         FIG. 4  shows an example of a gas shower according to the first embodiment; 
         FIGS. 5 and 6  show examples of simulation results in a first comparative example; 
         FIGS. 7 and 8  show examples of simulation results in the first embodiment; 
         FIG. 9  shows an example of a plasma processing system according to a second embodiment of the present disclosure; 
         FIG. 10  shows an example of a configuration of a nozzle in a second comparative example; 
         FIG. 11  shows an example of a configuration of a gas shower in a first modification of the second embodiment; 
         FIG. 12  shows an example of a simulation result in the second comparative example; 
         FIG. 13  shows an example of a simulation result in the first modification of the second embodiment; 
         FIGS. 14 to 16  show examples of simulation results in the second comparative example; 
         FIGS. 17 to 19  show examples of simulation results in the first modification of the second embodiment; 
         FIG. 20  shows an example of a simulation result of an electromagnetic field in a third comparative example; 
         FIG. 21  shows an example of a simulation result of an electromagnetic field in a second modification of the second embodiment; 
         FIG. 22  shows an example of a simulation result of an electromagnetic field in the third comparative example; and 
         FIG. 23  shows an example of a simulation result of an electromagnetic field in the second modification of the second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, embodiments of a plasma processing apparatus of the present disclosure will be described in detail with reference to the accompanying drawings. The present disclosure is not limited by the following embodiments. 
     In an inductively coupled plasma (ICP) type plasma processing apparatus, an induced electric field is generated in a processing chamber using a coil-shaped antenna. Therefore, a gas injector for introducing a processing gas into the processing chamber is disposed at a central portion of a ceiling plate of the processing chamber while avoiding a portion where the antenna is disposed. However, the gas injector introduces the processing gas into a small-diameter area of the processing chamber. Thus, the distribution of the gas that is introduced downward has a convex shape at a central portion of a substrate, and is difficult to control. Accordingly, it is desireable to improve the controllability of the gas distribution on the substrate. 
     (Configuration of the Plasma Processing System  1  According to the First Embodiment) 
       FIG. 1  shows an example of a plasma processing system according to the first embodiment of the present disclosure. As shown in  FIG. 1 , in one embodiment, the plasma processing system  1  includes a plasma processing apparatus  10  and a controller  100 . The plasma processing apparatus  10  includes a plasma processing chamber  11 , a gas supply unit  50 , a radio frequency (RF) power supply unit  300 , and a gas exhaust system  15 . The plasma processing apparatus  10  further includes a substrate support  20 , a gas shower  41 , and an antenna  62 . The substrate support  20  is disposed at a lower area of a plasma processing space  11   s  in the plasma processing chamber  11 . The plasma processing space  11   s  is defined by a sidewall and a dielectric window  61  (ceiling plate) of the plasma processing chamber  11 . The gas shower  41  is disposed above the substrate support  20  and is fitted to a central opening  61   a  of the dielectric window  61 . The antenna  62  is disposed on or above the plasma processing chamber  11  (the dielectric window  61 ). 
     The substrate support  20  is configured to support the substrate W in the plasma processing space  11   s . In one embodiment, the substrate support  20  includes a lower electrode  21 , an electrostatic chuck  22 , and an edge ring  23 . The electrostatic chuck  22  is disposed on the lower electrode  21  and is configured to support the substrate W on an upper surface of the electrostatic chuck  22 . The lower electrode  21  functions as a bias electrode. The edge ring  23  is disposed to surround the substrate W on an upper surface of a peripheral portion of the lower electrode  21 . Although it is not shown, in one embodiment, the substrate support  20  may include a temperature control module configured to adjust at least one of the electrostatic chuck  22  and the substrate W to a target temperature. The temperature control module may include a heater, a flow channel, or a combination thereof. A temperature control fluid such as a coolant or a heat transfer gas flows through the flow channel. 
     The gas shower  41  is configured to supply one or more processing gases from the gas supply unit  50  to the plasma processing space  11   s . In one embodiment, the gas shower  41  has a gas inlet  42 , a gas diffusion space  43 , and a plurality of bottom gas injection holes  46  and  47  and a plurality of side gas injection holes  48 . The gas shower  41  has a structure in which a horizontal dimension is greater than a vertical dimension. The bottom gas injection holes  46  and  47  and the side gas injection holes  48  are in fluid communication with the gas supply unit  50  and the gas diffusion space  43 . Further, the bottom gas injection holes  46  and  47  and the side gas injection holes  48  are in fluid communication with the gas diffusion space  43  and the plasma processing space  11   s . In one embodiment, the gas shower  41  is configured to supply one or more processing gases from the gas inlet  42  to the plasma processing space  11   s  through the gas diffusion space  43 , the bottom gas injection holes  46  and  47 , and the side gas injection holes  48 . 
     The gas supply unit  50  may include one or more gas sources  51 , one or more flow controllers  52 , a valve  53 , a line  54 , and a flow splitter (gas flow distributor)  55 . In one embodiment, the gas supply unit  50  is configured to supply one or more processing gases from the corresponding gas sources  51  to the gas shower  41  through the corresponding flow controllers  52 , and the valve  53 , the line  54 , and the flow splitter  55 . Each of the flow controllers  52  may include, for example, a mass flow controller (MFC) or a pressure-control type flow controller. 
     The RF power supply unit  300  is configured to supply an RF power, e.g., one or more RF signals, to the lower electrode  21  and the antenna  62 . Accordingly, plasma is generated from one or more processing gases supplied to the plasma processing space  11   s . Therefore, the RF power supply unit  300  can function as at least a part of a plasma generation unit configured to generate plasma from one or more processing gases in the plasma processing chamber. In one embodiment, the RF power supply unit  300  includes a first RF power supply  71  and a second RF power supply  30 . 
     The first RF power supply  71  includes a first RF generator and a first matching circuit. In one embodiment, the first RF power supply  71  is configured to supply a first RF signal from the first RF generator to the antenna  62  through the first matching circuit. In one embodiment, the first RF signal is a RF source signal having a frequency within a range of 27 MHz to 100 MHz. 
     The second RF power supply  30  includes a second RF generator and a second matching circuit. In one embodiment, the second RF power supply  30  is configured to supply a second RF signal from the second RF generator to the lower electrode  21  through the second matching circuit. In one embodiment, the second RF signal is a RF bias signal having a frequency within a range of 400 kHz to 13.56 MHz. 
     The antenna  62  includes an outer coil  621  and an inner coil  622  that are arranged to be coaxial with the gas shower  41 . The inner coil  622  is disposed around the gas shower  41  to surround the gas shower  41 . The outer coil  621  is disposed around the inner coil  622  to surround the inner coil  622 . The outer coil  621  functions as a primary coil to which the first RF power supply  71  is connected. In one embodiment, the outer coil  621  is a planar coil and has a substantially circular spiral shape. The inner coil  622  functions as a secondary coil that is inductively coupled to the primary coil. In other words, the inner coil  622  is not connected to the first RF power supply  71 . In one embodiment, the inner coil  622  is a planar coil and has a substantially circular ring shape. In one embodiment, the inner coil  622  is connected to a variable capacitor, and a direction or a magnitude of a current flowing through the inner coil  622  is controlled by controlling a capacitance of the variable capacitor. The outer coil  621  and the inner coil  622  may be arranged at the same height or at different heights. In one embodiment, the inner coil  622  is located lower than the outer coil  621 . 
     The gas exhaust system  15  may be connected to, e.g., a gas exhaust port  13  disposed at a bottom portion of the plasma processing chamber  11 . The gas exhaust system  15  may include a pressure valve and a vacuum pump. The vacuum pump may include a turbo molecular pump, a roughing pump, or a combination thereof. 
     In one embodiment, the controller  100  processes computer-executable instructions for causing the plasma processing apparatus  10  to perform various processes described in the present disclosure. The controller  100  may be configured to control the individual components of the plasma processing apparatus  10  to perform various processes described herein. In one embodiment, the controller  100  may be partially or entirely included in the plasma processing apparatus  10 . The controller  100  may include, e.g., a computer  101 . The computer  101  may include, e.g., a central processing unit (CPU)  102 , a storage unit  103 , and a communication interface  104 . The CPU  102  may be configured to perform various control operations based on programs stored in the storage unit  103 . The storage unit  103  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  104  may communicate with the plasma processing apparatus  10  through a communication line such as a local area network (LAN) or the like. 
     (Structure of the Antenna  62 ) 
     Next, the antenna  62  will be described in detail with reference to  FIGS. 2 and 3 .  FIG. 2  is a schematic perspective view showing an example of the antenna according to the first embodiment.  FIG. 3  shows an example of an arrangement of the inner coil and the outer coil according to the first embodiment. As shown in  FIGS. 2 and 3 , the antenna  62  is an example of an antenna assembly disposed above the dielectric window  61 , and the antenna assembly includes a central region  62   a , a first peripheral region  62   b  and a second peripheral region  62   c . The first peripheral region  62   b  surrounds the central region  62   a , and the second peripheral region  62   c  surrounds the first peripheral region  62   b . The central region  62   a  and the first peripheral region  62   b  vertically overlap the central opening  61   a  of the dielectric window  61 . Further, the gas shower  41  is disposed in the central opening  61   a.    
     The outer coil  621  is wound two or more turns in a substantially circular spiral shape. The outer coil  621  is disposed in the second peripheral region  62   c  such that the central axis of the outer shape of the outer coil  621  coincides with the Z-axis. The inner coil  622  is formed in, e.g., a substantially circular ring shape. The inner coil  622  is disposed above the dielectric window  61  such that the central axis of the inner coil  622  coincides with the Z-axis. Further, the inner coil  622  is disposed at a position in the first peripheral region  62   b  to allow the inner coil  622  to vertically overlap with an outer peripheral portion of the gas shower  41  or at a position in the first peripheral region  62   b  to allow the inner coil  622  to be disposed at an outer side of the outer peripheral portion of the gas shower  41 . Since it is considered that abnormal discharge occurs in the gas diffusion space  43  when a horizontal dimension of the gas shower  41  increases up to a position where the gas shower  41  is vertically overlapped with the outer coil  621 , it is preferable that the gas shower  41  is arranged not to be overlapped with the outer coil  621 . 
     The outer coil  621  and the inner coil  622  are planar coils and are arranged above a bottom surface of the dielectric window  61 , which is the boundary surface with the plasma processing space  11   s , to be substantially parallel to a surface of the substrate W placed on the electrostatic chuck  22 . Further, a distance between the outer coil  621  and the bottom surface of the dielectric window  61  is different from a distance between the inner coil  622  and the bottom surface of the dielectric window  61 . For example, the distance between the inner coil  622  and the bottom surface of the dielectric window  61  is shorter than the distance between the outer coil  621  and the bottom surface of the dielectric window  61 . In another example, the distance between the outer coil  621  and an upper surface of the dielectric window  61  and the distance between the inner coil  622  and the upper surface of the dielectric window  61  may be the same. In still another example, the distance between the outer coil  621  and the upper surface of the dielectric window  61  may be longer than the distance between the inner coil  622  and the upper surface of the dielectric window  61 . In still another example, the distance between the outer coil  621  and the bottom surface of the dielectric window  61  and the distance between the inner coil  622  and the bottom surface of the dielectric window  61  may be independently changed by a driving unit (not shown).  FIG. 3  shows an example of the arrangement of the inner coil  622  and the outer coil  621  when viewed from the Z-axis direction. The inner coil  622  has a circular shape along the outer peripheral portion of the gas shower  41  and is disposed in the first peripheral region  62   b  such that the center of the circle coincides with the Z-axis. 
     The outer coil  621  includes a wire having two open ends. The first RF power supply  71  is connected to a central point (first contact point) of the wire forming the outer coil  621  or to a vicinity (second contact point) of the central point, and a RF source signal (RF power) is supplied from the first RF power supply unit  71  to the outer coil  621 . The vicinity of the central point of the wire forming the outer coil  621  is grounded. The outer coil  621  is configured to resonate at a frequency having a wavelength that is half of a wavelength z of the RF source signal supplied from the first RF power supply  71 . In other words, the outer coil  621  functions as a planar helical resonator. A voltage generated in the wire forming the outer coil  621  is distributed such that it becomes the minimum near the central point of the wire and becomes the maximum at both ends of the wire. Further, a current generated in the wire forming the outer coil  621  is distributed such that it becomes the maximum near the central point of the wire and becomes the minimum at both ends of the wire. The frequency and the power of the RF source signal supplied from the first RF power supply  71  to the outer coil  621  may be changed. The frequency and the power of the RF source signal supplied from the first RF power supply  71  to the outer coil  621  are controlled by the controller  100 . 
     Both ends of a wire forming the inner coil  622  are connected to each other through a capacitor  623 . In other words, the inner coil  622  has a wire having two ends and the capacitor  623  connected to the two ends. The capacitor  623  is a variable capacitor. The capacitor  623  may be a capacitor having a fixed capacitance. The inner coil  622  is inductively coupled with the outer coil  621 . The current flows through the inner coil  622  in a direction to cancel a magnetic field generated by the current flowing through the outer coil  621 . It is possible to control the direction or the magnitude of the current flowing through the inner coil  622  with respect to the current flowing through the outer coil  621  by controlling the capacitance of the capacitor  623 . The capacitance of the capacitor  623  is controlled by the controller  100 . 
     A magnetic field is generated in the Z-axis direction by the current flowing through the outer coil  621  and the current flowing through the inner coil  622 , and an induced electric field is generated in the plasma processing chamber  11  by the magnetic field. Due to the induced electric field generated in the plasma processing chamber  11 , the processing gas supplied from the gas shower  41  into the plasma processing chamber  11  is turned into plasma. Then, predetermined processing such as etching is performed on the substrate W on the electrostatic chuck  22  by ions or active species contained in the plasma. 
     (Gas Injection Holes of the Gas Shower  41 ) 
     Next, the gas injection holes of the gas shower  41  will be described with reference to  FIG. 4 .  FIG. 4  shows an example of the gas shower according to the first embodiment. As shown in  FIG. 4 , the plurality of bottom gas injection holes  46  and  47  are formed in a bottom portion  44  of the gas shower  41 . The bottom gas injection holes  46  include, e.g., a plurality of bottom gas injection holes  46   a ,  46   b , and  46   c , arranged on the respective circumferences of concentric circles having different diameters. The bottom gas injection holes  47  are arranged on the circumference of a circle concentric with the circles respectively formed by the bottom gas injection holes  46   a ,  46   b , and  46   c , while being disposed at the outer side of the bottom gas injection holes  46   a ,  46   b , and  46   c . In other words, the bottom gas injection holes  46   a ,  46   b ,  46   c , and  47  are arranged on the respective circumferences of concentric circles at equal intervals, for example. The bottom gas injection holes  46   a ,  46   b , and  46   c  are formed to extend in the vertical direction, i.e., in the Z-axis direction of the plasma processing chamber  11 . The bottom gas injection holes  47  are formed to extend obliquely toward the outer peripheral side, for example. In  FIG. 1 , the bottom gas injection holes  46   c  are omitted. 
     Further, the plurality of side gas injection holes  48  are formed at a side portion  45  of the gas shower  41 . The side gas injection holes  48  are arranged on the circumference along the side portion  45  of the gas shower  41  at equal intervals, for example. The side gas injection holes  48  are formed to extend in a horizontal direction, i.e., in a direction perpendicular to the side portion  45 . The bottom gas injection holes  46  and  47  and the side gas injection holes  48  may be arranged in a different manner, for example, in multiple rows or in a zigzag shape, and are not necessarily arranged at equal intervals. Further, the gas shower  41  is disposed such that the side gas injection holes  48  are exposed to the plasma processing space  11   s.    
     Further, the bottom gas injection holes  46  and  47  have a first hole diameter of 0.05 mm to 1.5 mm and the side gas injection holes  48  have a second hole diameter of 0.05 mm to 1.5 mm, for example. The first hole diameter may be equal to or different from the second hole diameter. 
     (Simulation Result) 
     Next, the simulation results of the first embodiment and a first comparative example will be described with reference to  FIGS. 5 to 8 . In the first comparative example, the simulation was performed on an ICP type plasma processing apparatus in which a gas injector is disposed at the central portion of the dielectric window  61  of the plasma processing chamber  11 . The gas injector has a structure in which a vertical dimension is greater than a horizontal dimension. 
     &lt;Simulation Conditions&gt; 
     Chamber pressure: 6.67 Pa (50 mTorr) 
     Processing gas: Ar=500 sccm 
       FIGS. 5 and 6  show examples of the simulation results in the first comparative example.  FIGS. 7 and 8  show examples of the simulation results in the first embodiment.  FIGS. 5 and 6  show the distribution of a pressure and the distribution of a flow velocity on the substrate W for each distance between the substrate W and the dielectric window  61  in the simulation of the first comparative example.  FIGS. 7 and 8  show the distribution of a pressure and the distribution of a flow velocity on the substrate W for each distance between the substrate W and the dielectric window  61  in the simulation of the first embodiment. The distances between the substrate W and the dielectric window  61  are represented by “Gap H” corresponding to “high,” “Gap M” corresponding to “middle,” and “Gap L” corresponding to “low” in  FIGS. 5 to 8 . 
     According to the simulation results for the pressure on the substrate W between the first comparative example and the first embodiment, in the first comparative example shown in  FIG. 5 , the pressure is highest near the center of the substrate W and decreases toward the peripheral portion of the substrate W. In other words, the pressure distribution has a convex shape at the central portion. In contrast, in the first embodiment shown in  FIG. 7 , the pressure distribution is substantially uniform from the center of the substrate W to the peripheral portion of the substrate W. 
     According to the simulation results for the flow velocity on the substrate W between the first comparative example and the first embodiment, in the first comparative example shown in  FIG. 6 , the flow velocity is highest near a portion separated from the center of the substrate W by a distance of about 3 cm to 4 cm and decreases toward the peripheral portion of the substrate W. In other words, the flow velocity distribution has a convex shape at the central portion. In contrast, in the first embodiment shown in  FIG. 8 , the flow velocity distribution is substantially uniform from the center of the substrate W to the peripheral portion of the substrate W. In other words, in the first embodiment, the tendency in which the pressure distribution and the flow velocity distribution have a convex shape at the central portion can be suppressed using the gas shower  41 . In other words, the controllability of the gas distribution on the substrate W can be improved. 
     (Configuration of the Plasma Processing System  2  According to the Second Embodiment) 
     In the first embodiment, the gas shower  41  has one gas diffusion space  43 . However, it is also possible to divide the gas diffusion space  43  into a plurality of regions to distribute and control the flow amount of the processing gas to be supplied to each region. Such a case will be described as the second embodiment. Further, such a structure is referred to as a “radial distribution control (RDC) structure.” Like reference numerals will be given to like parts as those of the first embodiment, and the redundant description on the same configuration and operation will be omitted. 
       FIG. 9  shows an example of a plasma processing system according to the second embodiment of the present disclosure. The plasma processing system  2  shown in  FIG. 9  is different from the plasma processing system  1  according to the first embodiment in that it includes a plasma processing apparatus  10   a  instead of the plasma processing apparatus  10 . Further, the plasma processing apparatus  10   a  is different from the plasma processing apparatus  10  in that it includes a gas shower  81  instead of the gas shower  41 . 
     The gas shower  81  has gas inlets  82   a  and  82   b , gas diffusion spaces  83   a  and  83   b , and a plurality of bottom gas injection holes  86  and  87  and a plurality of side gas injection holes  88 . The arrangement of the bottom gas injection holes  86  and  87  and the side gas injection holes  88  of the gas shower  81  may be the same as the arrangement of the gas shower  41  shown in  FIG. 4 , for example. In other words, the arrangement of the bottom gas injection holes  86   a ,  86   b , and  87  and the side gas injection holes  88  corresponds to the arrangement of the bottom gas injection holes  46   a ,  46   b , and  47  and the side gas injection holes  48  shown in  FIG. 4 . In the gas shower  81  shown in  FIG. 9 , the bottom gas injection holes corresponding to the bottom gas injection holes  46   c  of the gas shower  41  are not illustrated for simplicity of description. However, the gas shower  81  may have the bottom gas injection holes corresponding to the bottom gas injection holes  46   c  of the gas shower  41 . 
     The bottom gas injection holes  86  are formed in a bottom portion  84  of the gas shower  81 . The bottom gas injection holes  86  include, e.g., a plurality of bottom gas injection holes  86   a  and  86   b  arranged on the respective circumferences of concentric circles having different diameters. The bottom gas injection holes  86   a  and  86   b  are in fluid communication with the gas diffusion space  83   a  and the gas supply unit  50  connected thereto through the gas inlet  82   a . Further, the bottom gas injection holes  86   a  and  86   b  are in fluid communication with the gas diffusion space  83   a  and the plasma processing space  11   s . The bottom gas injection holes  86   a  and  86   b  are formed to extend in the vertical direction, i.e., in the Z-axis direction of the plasma processing chamber  11 . 
     The bottom gas injection holes  87  are formed in the bottom portion  84  of the gas shower  81 . The bottom gas injection holes  87  are arranged on the circumference of a circle concentric with other circles respectively formed by the bottom gas injection holes  86   a  and  86   b  while being disposed at the outer side of the bottom gas injection holes  86   a  and  86   b . The bottom gas injection holes  87  are in fluid communication with the gas diffusion space  83   b  and the gas supply unit  50  connected thereto through the gas inlet  82   b . Further, the bottom gas injection holes  87  are in fluid communication with the gas diffusion space  83   b  and the plasma processing space  11   s . The bottom gas injection holes  87  are formed to extend obliquely toward the outer peripheral side, for example. 
     The side gas injection holes  88  are arranged on the circumference of the gas shower  81  along a side portion  85  of the gas shower  81  at equal intervals, for example. The side gas injection holes  88  are in fluid communication with the gas diffusion space  83   b  and the gas supply unit  50  connected thereto through the gas inlet  82   b . Further, the side gas injection holes  88  are in fluid communication with the gas diffusion space  83   b  and the plasma processing space  11   s . The side gas injection holes  88  are formed to extend in the horizontal direction, i.e., in a direction perpendicular to the side portion  85 . Similar to the first embodiment, the bottom gas injection holes  86 ,  87  and the side gas injection holes  88  may be arranged in a different manner, for example, in multiple rows or in a zigzag shape, and are not necessarily arranged at equal intervals. Similar to the gas shower  41 , the gas shower  81  is disposed such that the side gas injection holes  88  are exposed to the plasma processing space  11   s.    
     The gas inlets  82   a  and  82   b  are respectively connected to the flow splitter  55  through lines  56   a  and  56   b . The flow splitter  55  is configured to distribute and control the flow rate of the processing gas. The flow splitter  55  may be a gas box or the like as long as it is configured to change the flow ratio of the processing gas supplied to the gas diffusion spaces  83   a  and  83   b.    
     (Simulation Results) 
     Next, the simulation results of a second comparative example and a first modification of the second embodiment will be described with reference to  FIGS. 10 to 19 . In the second comparative example, the simulation was performed on an ICP type plasma processing apparatus in which a gas injector is disposed at the center of the dielectric window  61  of the plasma processing chamber  11 . 
     First, the structures of the gas injector and the gas shower used in the simulation of the second comparative example and the first modification of the second embodiment will be described with reference to  FIGS. 10 and 11 .  FIG. 10  shows an example of a configuration of a nozzle in the second comparative example. As shown in  FIG. 10 , a gas injector  200  of the second comparative example has two nozzle systems in the vertical direction (center) and the horizontal direction (side), and the processing gas is supplied to the plasma processing space  11   s  through the two nozzle systems. 
       FIG. 11  shows an example of a configuration of a gas shower in the first modification of the second embodiment. As shown in  FIG. 11 , the gas shower  201  of the first modification of the second embodiment has the same RDC structure as that of the gas shower  81 , but is different from the gas shower  81  in that it has four gas diffusion spaces so that the processing gas is supplied through four gas injection hole systems. That is, the gas shower  201  is configured to supply the processing gas to the plasma processing space  11   s  through the four (center, middle, edge, and side) gas injection hole systems. 
     Specifically, the gas shower  201  has a first diffusion space disposed at the center of the gas shower  201 , a second diffusion space surrounding the first diffusion space, a third diffusion space surrounding the second diffusion space, and a fourth diffusion space surrounding the third diffusion space. Further, the first diffusion space, the second diffusion space, the third diffusion space, and the fourth diffusion space are formed in the gas shower  201  but are not in fluid communication with each other. The gas injection holes of the gas shower  201  are the same as those of the gas shower  41  shown in  FIG. 4  or the gas shower  81 . In the case of applying the gas injection holes of the gas shower  81  to the gas injection holes of the gas shower  201 , the bottom gas injection holes  86   a  (center holes) are in fluid communication with the first diffusion space. Similarly, the bottom gas injection holes  86   b  (middle holes) are in fluid communication with the second diffusion space, and the bottom gas injection holes  87  (edge holes) are in fluid communication with the third diffusion space. The side gas injection holes  88  (side holes) are in fluid communication with the fourth diffusion space. 
     In other words, in the gas shower  201 , the bottom gas injection holes  86   a  and  86   b  being in fluid communication with the same gas diffusion space  83   a  in the gas shower  81  are in fluid communication with different gas diffusion spaces, i.e., the first diffusion space and the second diffusion space. Similarly, in the gas shower  201 , the bottom gas injection holes  87  and the side gas injection holes  88  being in fluid communication with the same gas diffusion space  83   b  in the gas shower  81  are in fluid communication with different gas diffusion spaces, i.e., the third diffusion space and the fourth diffusion space. 
     Next, the simulation result of the gas amount on the substrate W when using the gas injector  200  and the gas shower  201  will be compared with reference to  FIGS. 12 and 13 . 
     &lt;Simulation Conditions&gt; 
     Chamber pressure: 6.67 Pa (50 m Torr) 
     Main gas: C 4 F 8 =200 sccm 
     Sub-gas: Ar=50 sccm 
     (Total Flow Amount of Four Systems in the Gas Shower  201 ) 
       FIG. 12  shows an example of the simulation result in the second comparative Example.  FIG. 13  shows an example of the simulation result in the first modification of the second embodiment.  FIG. 12  shows the gas amount distribution of the main gas (C 4 F 8 ) on the substrate W in the simulation of the second comparative example in which the main gas is supplied from each of the center nozzle and the side nozzle of the gas injector  200 .  FIG. 13  shows the gas amount distribution of the main gas (C 4 F 8 ) on the substrate W in the simulation of the first modification of the second embodiment in which the main gas is supplied from each set of the center gas injection holes, the middle gas injection holes, the edge gas injection holes, and the side gas injection holes of the gas shower  201 . Further, in order to prevent gas backflow, the sub-gas is supplied to the nozzle and the set(s) of the gas injection holes to which the main gas is not supplied. In the vertical axis of  FIGS. 12 and 13 , “1” indicates the case where the amount of C 4 F 8  gas is 100% and “0” indicates the case where the amount of C 4 F 8  gas is 0%. 
     According to the comparison of the gas amount distribution of the main gas (C 4 F 8 ) on the substrate W between the second comparative example and the first modification of the second embodiment, in the second comparative example shown in  FIG. 12 , the difference in the amount of the main gas between the center and the edge is about 0.005 (0.5%) near the center of the substrate W and decreases toward the peripheral portion of the substrate W. In other words, the gas injector  200  has poor controllability for the gas amount of the main gas. In contrast, in the first modification of the second embodiment shown in  FIG. 13 , the difference in the amount of the main gas between the center, the middle, the edge, and the side is within a range of about 0.007 (0.7%) to about 0.028 (2.8%) near the center of the substrate W, and is within a range of about 0.002 (0.2%) to about 0.015 (1.5%) at the peripheral portion of the substrate W. 
     In other words, the gas shower  201  has a wider range of controlling the amount of the main gas, and thus has improved controllability. For example, as shown in  FIG. 13 , in the case of supplying the main gas from the center of the gas shower  201 , it is possible to control the gas amount to be decreased toward the peripheral portion of the substrate W, as in the case of supplying the main gas from the center of the gas injector  200 . On the other hand, in the case of supplying the main gas from the middle, the edge or the side of the gas shower  201 , it is possible to control the gas amount to be increased toward the peripheral portion of the substrate W. In other words, in the first modification of the second embodiment, the controllability of the gas distribution on the substrate W can be further improved. 
     (Control of Flow Ratio) 
     The gas shower  201  can have further improved controllability by supplying the main gas from the plurality of gas injection holes at a controlled flow ratio. An example of controlling the flow ratio of the gas injection holes using the gas shower  201  will be described in comparison with the case of using the gas injector  200  with reference to  FIGS. 14 to 19 . 
       FIGS. 14 to 16  show examples of the simulation results in the second comparative example.  FIGS. 17 to 19  show examples of the simulation results in the first modification of the second embodiment.  FIGS. 14 to 16  show the distribution of the pressure, the flow velocity, and the amount of C 4 F e  gas on the substrate W in the simulation of the second comparative example.  FIGS. 14 to 16  show the case of supplying C 4 F 8  gas of 200 sccm as the main gas from the center of the gas injector  200  and supplying Ar gas of 50 sccm as the sub-gas from the side (pattern A) of the gas injector  200 , and the opposite case where the gas supplied from the center and the gas supplied from the side are switched with each other (Pattern B). 
     &lt;Simulation Conditions for the Second Comparative Example&gt; 
     Chamber pressure: 6.67 Pa (50 mTorr) 
     Main gas (C 4 F 8 ): Center=200 sccm (Pattern A)
         Side=200 sccm (Pattern B)       

     Sub-gas (Ar): Side=50 sccm (Pattern A)
         Center=50 sccm (Pattern B)       

       FIGS. 17 to 19  show the distribution of the pressure, the flow velocity, and the amount of C 4 F 8  gas on the substrate W in the simulation of the first modification of the second embodiment. In  FIGS. 17 to 19 , C 4 F 8  gas was supplied as the main gas at a total flow rate of 200 sccm in three patterns, i.e., only from the center gas injection holes of the gas shower  201  (pattern C), from the center and the middle gas injection holes of the gas shower  201  (pattern D), and from the center, the middle, and the edge gas injection holes of the gas shower  201  (pattern E). Ar gas was supplied as the sub-gas at a total flow rate of 50 sccm from the other gas injection holes. 
     &lt;Simulation Conditions for the First Modification of the Second Embodiment&gt; 
     Chamber pressure: 6.67 Pa (50 mTorr) 
     Main gas (C 4 F 8 ):
         Center=200 sccm (Pattern C)   Center/Middle=100/100 sccm (Pattern D)   Center/Middle/Edge=66.7/66.7/66.7 sccm (Pattern E) Sub-gas (Ar):   Middle/Edge/Side=16.7/16.7/16.7 sccm (Pattern C)   Edge/Side=25/25 sccm (Pattern D)   Side=50 sccm (Pattern E)       

     According to the simulation results for the pressure on the substrate W between the second comparative example and the first modification of the second embodiment, in the pattern A of the second comparative example shown in  FIG. 14 , the pressure is highest near the center of the substrate W and decreases toward a portion separated from the center of the substrate W by a distance of about 5 cm. In other words, the pressure distribution has a convex shape at the central portion. In the pattern B, the pressure distribution is substantially uniform from the center of the substrate W to the peripheral portion of the substrate W. In contrast, in the first modification of the second embodiment shown in  FIG. 17 , the pattern C has the same distribution as that of the pattern A of the second comparative example, and the pattern E has a substantially uniform pressure distribution from the center of the substrate W to the peripheral portion of the substrate W. In the pattern D, the pressure near the center of the substrate W is slightly higher than that in the pattern E. 
     According to the simulation results for the flow velocity on the substrate W between the second comparative example and the first modification of the second embodiment, in the pattern A of the second comparative example shown in  FIG. 15 , the flow velocity decreases at a substantially uniform rate from the center of the substrate W to the peripheral portion of the substrate W. In the pattern B, the flow velocity distribution is substantially uniform from the center of the substrate W to the peripheral portion of the substrate W. In contrast, in the first modification of the second embodiment shown in  FIG. 18 , the pattern C has a distribution similar to that of the pattern A of the second comparative example. In the patterns D and E, the distribution has a slightly convex shape near a portion separated from the center of the substrate W by a distance of about 4 cm. 
     According to the simulation results for the gas amount distribution of the main gas (C 4 F 8 ) between the second comparative example and the first modification of the second embodiment, in the second comparative example shown in  FIG. 16 , the difference between the patterns A and B is about 0.005 (0.5%) near the center of the substrate W and decreases toward the peripheral portion of the substrate W. In contrast, in the first modification of the second embodiment shown in  FIG. 19 , the difference in the amount of the main gas between the patterns D and E is about 0.023 (2.3%) near the center of the substrate W. Further, the difference in the amount of the main gas between the patterns D and E is about 0.02 (2%) near the peripheral portion of the substrate W. The amount of the main gas in the pattern C has a substantially intermediate value between those in the patterns D and E from the center of the substrate W to the peripheral portion of the substrate W. In other words, the gas shower  201  allows the fine control of the pressure, the flow velocity, and the gas amount of the main gas on the substrate W by controlling the flow ratio. 
     (Simulation Results of the Electromagnetic Field) 
     Next, the influence on the electromagnetic field distribution in a third comparative example and a second modification of the second embodiment will be described with reference to  FIGS. 20 to 23 . As described in the structure of the antenna  62 , the outer coil  621  functions as a resonator and generates strong electric and magnetic fields. In contrast, the inner coil  622  is not connected to the first RF power supply  71  and has a closed loop. Further, the inner coil  622  is inductively coupled to the outer coil  621  to generate an induced electromotive force in the antenna. In other words, the inner coil  622  is an example of an absorbing coil. A variable capacitor  623  is connected to the inner coil  622 , and the amount of current flowing through the inner coil  622  (hereinafter, also referred to as “amount of drawn current”) can be controlled by adjusting an impedance. 
       FIGS. 20 and 22  show examples of the simulation results of an electromagnetic field in the third comparative example.  FIGS. 21 and 23  show examples of the simulation results of an electromagnetic field in the second modification of the second embodiment. In the third comparative example, an outer coil  211  and an inner coil  212  are used. In the second modification of the second embodiment, an outer coil  213  and an inner coil  214  are used instead of the outer coil  621  and the inner coil  622 . In the third comparative example and the second modification of the second embodiment, the distances between the outer coil  211  and the bottom surface of the dielectric window and between the inner coil  212  and the bottom surface of the dielectric window are the same as the distances between the outer coil  213  and the bottom surface of the dielectric window and between the inner coil  214  and the bottom surface of the dielectric window. 
       FIGS. 20 and 21  show the electromagnetic field distribution of the plasma processing space  11   s  in the case where the amount of drawn current to the inner coils  212  and  214  is “0.” The electromagnetic field distribution of the plasma processing space  11   s  in the second modification of the second embodiment shown in  FIG. 21  is substantially the same as the electromagnetic field distribution of the plasma processing space  11   s  in the third comparative example shown in  FIG. 29 . In other words, when the amount of drawn current is “0,” the electromagnetic field distribution is hardly affected even when the gas injector is changed to the gas shower. 
       FIGS. 22 and 23  show the electromagnetic field distribution of the plasma processing space  11   s  in the case where the amount of drawn current to the inner coils  212  and  214  is the maximum. The electromagnetic field distribution of the plasma processing space  11   s  in the second modification of the second embodiment shown in  FIG. 23  is substantially the same as the electromagnetic field distribution of the plasma processing space  11   s  in the third comparative example shown in  FIG. 22 . In other words, when the amount of drawn current is the maximum, the electromagnetic field distribution is hardly affected even when the gas injector is changed to the gas shower. 
     As described above, in accordance with the first embodiment, the plasma processing apparatus  10  includes the chamber (the plasma processing chamber  11 ), the antenna assembly (the antenna  62 ), the primary coil (the outer coil  621 ), the RF power supply (the first RF power supply  71 ), and the gas shower  41 . The chamber includes the sidewall and the ceiling plate (the dielectric window  61 ) having the central opening  61   a , and the sidewall and the ceiling plate define the plasma processing space  11   s . The antenna assembly is disposed above the ceiling plate. The antenna assembly includes the central region  62   a , the first peripheral region  62   b , and the second peripheral region  62   c . The first peripheral region  62   b  surrounds the central region  62   a , and the second peripheral region  62   c  surrounds the first peripheral region  62   b . The central region  62   a  and the first peripheral region  62   b  vertically overlap the central opening  61   a . The primary coil is disposed in the second peripheral region  62   c . The RF power supply is configured to supply a RF signal to the primary coil. The gas shower  41  is disposed at the central opening  61   a  and has the bottom portion  44  exposed to the plasma processing space  11   s . The bottom portion  44  has the plurality of bottom gas injection holes  46  and  47 . Accordingly, the controllability of the gas distribution on the substrate W can be improved. 
     Further, in accordance with the first embodiment, the plasma processing apparatus  10  further includes the secondary coil (the inner coil  622 ) disposed in the first peripheral region  62   b . The secondary coil is inductively coupled to the primary coil. Accordingly, plasma can be generated in the plasma processing space  11   s.    
     Further, in accordance with the first embodiment, the secondary coil has a wire having two ends and the capacitor  623  connected to the two ends. Accordingly, the direction or the magnitude of the current flowing through the inner coil  622  can be controlled. 
     Further, in accordance with the first embodiment, the gas shower  41  has the side portion  45 . At least a part of the side portion  45  is exposed to the plasma processing space  11   s , and the side portion  45  has the plurality of side gas injection holes  48 . Accordingly, the processing gas can be supplied in the horizontal direction. 
     Further, in accordance with the second embodiment, the plasma processing apparatus  10  further includes the gas distribution controller (the flow splitter  55 ). The gas shower  81  has the plurality of diffusion spaces (the gas diffusion spaces  83   a  and  83   b ). The gas distribution controller is configured to control the flow ratio of the gas distributed to the diffusion spaces, and each set of the bottom gas injection holes  86 , the bottom gas injection holes  87 , and the side gas injection holes  88  is in fluid communication with any one of the diffusion spaces. Accordingly, the controllability of the gas distribution on the substrate W can be further improved. 
     Further, in accordance with the second embodiment, the plasma processing apparatus  10  further includes the gas distribution controller (the flow splitter  55 ). The gas shower  201  includes the first diffusion space disposed at the center of the gas shower  201 , the second diffusion space surrounding the first diffusion space, the third diffusion space surrounding the second diffusion space, and the fourth diffusion space surrounding the third diffusion space. The gas distribution controller is configured to control the flow ratio of the gas distributed to the first diffusion space, the second diffusion space, the third diffusion space, and the fourth diffusion space. Each set of the bottom gas injection holes  86   a , the bottom gas injection holes  86   b , and the bottom gas injection holes  87  is in fluid communication with any one of the first diffusion space, the second diffusion space, and the third diffusion space. The set of the side gas injection holes  88  is in fluid communication with the fourth diffusion space. Accordingly, the controllability of the gas distribution on the substrate W can be further improved. 
     Further, in accordance with the second embodiment, the set of the bottom gas injection holes  87  being in fluid communication with the third diffusion space is formed to extend obliquely. Accordingly, the controllability of the gas distribution on the substrate W can be further improved. 
     In accordance with the above-described embodiments, the bottom gas injection holes  46 ,  47 ,  86 , and  87  have a first hole diameter of 0.05 mm to 1.5 mm and the side gas injection holes  48  and  88  have a second hole diameter of 0.05 mm to 1.5 mm. Accordingly, the flow rate or the flow velocity of the processing gas can be controlled by adjusting the conductance. 
     Further, in accordance with the above-described embodiments, the first hole diameter may be equal to the second hole diameter. Accordingly, it is possible to suppress the gas distribution having a convex shape at the central portion of the substrate W. 
     Further, in accordance with the above-described embodiments, the first hole diameter may be different from the second hole diameter. Accordingly, the flow rate or the flow velocity of the processing gas can be controlled by adjusting the conductance. 
     Further, in accordance with the above-described embodiments, the gas showers  41 ,  81 , and  210  have a horizontal dimension and a vertical dimension smaller than the horizontal dimension. Accordingly, the controllability of the gas distribution on the substrate W can be further improved. 
     Further, in accordance with the above-described embodiments, the primary coil has a wire having two open ends, and the wire has a first contact point connected to the RF power supply and a second contact point that is grounded. Accordingly, a RF signal can be supplied to the primary coil. 
     The presently disclosed embodiments are considered in all respect to be illustrative and not restrictive. The above-described embodiments can be embodied in various forms. Further, the above-described embodiments may be omitted, replaced, or changed in various forms without departing from the scope of the appended claims and the gist thereof. 
     In the above-described embodiments, the spiral coil having open ends is used as the antenna  62 . However, the antenna  62  is not limited thereto, and may be an antenna having another shape, for example, a coil having a wire whose one end is connected to the RF power supply and the other end is grounded, a loop-shaped coil, or the like. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the disclosures. Indeed, the embodiments described herein may be embodied in a variety of other forms. Furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made departing from the spirit of the disclosures. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the disclosures.