Patent Publication Number: US-2019189396-A1

Title: Plasma processing apparatus

Description:
CLAIM OF PRIORITY 
     The present application claims priority from Japanese Patent Application JP 2017-240725 filed on Dec. 15, 2017, the content of which is hereby incorporated by reference into this application. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a plasma processing apparatus that processes a substrate-shaped sample, such as a semiconductor wafer, mounted on the upper surface of a sample stage disposed in a processing room inside a vacuum chamber, with plasma formed in the processing room. Particularly, the present invention relates to a plasma processing apparatus including: a plate-shaped electrode to be supplied with power for forming plasma, disposed above the upper surface of a sample stage, the electrode being opposed to the upper surface of the sample stage; and a dielectric plate member included in the upper surface of a processing room, below the electrode, the dielectric plate member allowing an electric field for forming the plasma, to pass therethrough. 
     2. Description of the Related Art 
     Plasma processing is widely used in a manufacturing process of a semiconductor device, the plasma processing including: forming plasma in a processing room inside a decompressed chamber; and etching a film layer to be processed in a film structure including the film layer to be processed and a mask layer previously disposed on the surface of a substrate-shaped sample, such as a semiconductor wafer, disposed in the processing room. In order to form the plasma in the processing room, for example, radio-frequency power having a predetermined frequency is supplied to either of capacitive-coupled parallel plate electrodes including two electrodes of an upper electrode and a lower electrode opposed to each other above and below through a space for plasma formation in the processing room, to form an electric field in the space between the two electrodes, and then gas supplied in the space is excited and dissociated with the electric field, so that the plasma can be formed. The parallel-plate-type plasma processing apparatus attracts charged particles or active particles having high activity (radicals), such as ions, in the plasma formed in the space between the two electrodes, to the film structure on the upper surface of the wafer, to perform the processing. 
     Recent semiconductor devices have been progressively miniaturized in dimensions, and thus the precision in dimensions after etching processing is continuously in high demand. In order to achieve the demand, a technique of generating and processing high-density plasma with lower pressure, retaining a rate at which the dissociation appropriately occurs lower, has been considered, instead of a conventional technique in which the processing is performed with the dissociation rate of the particles of the gas in the processing room, higher. The power to be supplied in order to generate the plasma, typically has a radio-frequency band of 10 MHz or more in frequency, and a higher frequency has an advantage in high-density plasma generation. However, making the frequency higher reduces the wavelength of an electromagnetic wave, and thus an electric field distribution is ununiform in the plasma processing room. It has been known that the electric field distribution has a distribution having a center portion high that can be expressed with the superposition of Bessel functions. 
     The increase in the center portion in the electric field increases the electron density of the plasma, and thus uniformity degrades in an etching rate in-plane distribution. The degradation in the etching rate in-plane distribution drops productivity, and thus there is a need to improve the uniformity of the etching rate in the surface of the wafer with an increase in the frequency of the radio-frequency power. 
     A technique described in JP 2007-250838 A has been known as a conventional technique for resolving the problem. The present conventional technique relates to a plasma processing apparatus including: a discoid first electrode disposed in an upper part of a processing room inside a vacuum chamber, the discoid first electrode being to be supplied with radio-frequency power for plasma formation; and a second electrode disposed inside a sample stage on which a wafer is to be mounted, the second electrode being to be supplied with radio-frequency power, the sample stage being disposed below the processing room. The first electrode has a space at the joint between an electrode support and an electrode plate, the electrode support being disposed above the upper surface of the electrode plate, the electrode support being joined to the electrode plate. The height of the space at a center portion is larger than that of the space at a circumferential portion. This configuration relaxes the ununiformity of the intensity distribution of the electric field between the center portion and circumferential portion of the upper electrode, particularly, a convex distribution high at the center portion, so that the intensity distribution of the electric field in the direction from the center to the circumference, can further approximate uniformity. 
     Furthermore, according to Ken&#39;etsu Yokogawa et al.; Real time estimation and control oxide-etch rate distribution using plasma emission distribution measurement; Japanese Journal of Applied Physics, Vol. 47, No. 8, 2008, pp. 6854-6857, a technique of increasing power absorption efficiency in a region on the circumferential side of a wafer with an external magnetic field, to cause the distribution of electron density to be formed in a radial direction in a space between electrodes, to further approximate uniformity, has been proposed. According to the conventional technique, increase and decrease in the value of a current to be supplied to a coil, can adjust the intensity of the magnetic field to be formed by the coil, into a value in a desirable range. Thus, even when a condition in which plasma is to be formed varies, the intensity distribution of the plasma or the distribution of charged particles, such as electrons, can be adjusted in response to a variation in an electric field distribution in a processing room. This arrangement has an advantage in that the margin of the condition in which the plasma can be formed further approximating the uniformity, expands. 
     SUMMARY OF THE INVENTION 
     The conventional techniques have not sufficiently considered the following points, resulting in problems. 
     That is, the configuration in JP 2007-250838 A can make the electric field distribution uniform under a certain condition. However, the distribution of the electric field to be formed and the intensity distribution of the plasma or the distribution of charged particles to be strongly influenced by the electric field, vary in accordance with a condition in the processing room in which plasma is to be formed, including the pressure value in the processing room, the type of gas to be supplied for plasma formation or for wafer processing, the frequency value of the radio-frequency power, and the magnitude of the power. Thus, the conventional technique described in JP 2007-250838 A has, even in a wide-range condition in which plasma is to be formed, a limitation in causing the distribution of the electric field or the intensity distribution of the plasma in the processing room to approximate the uniformity. 
     The configuration disclosed in Ken&#39;etsu Yokogawa et al.; Real time estimation and control oxide-etch rate distribution using plasma emission distribution measurement; Japanese Journal of Applied Physics, Vol. 47, No. 8, 2008, pp. 6854-6857, has technical difficulty in causing the gradient of the electric field to completely agree with the gradient of the magnetic field in the radial direction of the electrodes disposed through the space in which the plasma is to be formed, and thus a region in which the electron density formed in the space is small is formed at an intermediate location between the center of the electrodes and the circumferential end thereof. Such a “drop” in the electron density causes the intensity of the plasma or the density of charged particles, such as ions, to locally drop in the space below the location of the occurrence of the drop. As a result, a processing characteristic at a location on the upper surface of the wafer disposed below the space, the wafer facing the plasma, the location being positioned below the “drop”, for example, an etching rate also drops in etching processing and ununiformity increases in the deviation of an after-processing processed shape from an expected shape in the in-plane direction of the upper surface of the wafer, and thus there is a risk that processing yield degrades. 
     The conventional techniques have not considered the problems. 
     An object of the present invention is to provide a plasma processing apparatus that inhibits ununiformity in the distribution of plasma and further improves processing yield. 
     The object of the present invention is achieved by a plasma processing apparatus including: a processing room disposed inside a vacuum chamber; a sample stage disposed inside the processing room, the sample stage having an upper surface on which a wafer to be processed is to be mounted; a dielectric discoid member disposed in an upper part of the processing room, the dielectric discoid member being opposed to the upper surface of the sample stage; a discoid upper electrode disposed having a side covered with the dielectric discoid member, the side facing the sample stage, the discoid upper electrode being to be supplied with first radio-frequency power for forming an electric field for forming plasma in the processing room; a coil disposed circumferentially above the processing room outside the vacuum chamber, the coil being configured to generate a magnetic field for forming the plasma; and a lower electrode disposed inside the sample stage, the lower electrode being to be supplied with second radio-frequency power for forming a bias potential on the wafer mounted on the sample stage. A ring-shaped recess and a metal ring-shaped member are provided between the discoid member and the upper electrode, the ring-shaped recess being formed on the discoid member, the metal ring-shaped member being embedded in the ring-shaped recess in contact with the upper electrode. 
     In addition, the object of the present invention is achieved by a plasma processing apparatus including: a processing room; a lower electrode unit provided to a lower portion of the processing room inside the processing room; an upper electrode unit provided inside the processing room, the upper electrode unit being opposed to the lower electrode unit; a vacuum exhaust unit configured to exhaust for a vacuum inside the processing room; a radio-frequency power applying unit configured to apply radio-frequency power to the upper electrode unit; a magnetic field generating unit provided outside the processing room, the magnetic field generating unit being configured to generate a magnetic field inside the processing room; a radio-frequency bias power applying unit configured to apply radio-frequency bias power to the lower electrode unit; and a gas supplying unit configured to supply processing gas from a side of the upper electrode unit into the processing room. The upper electrode unit includes: an antenna electrode unit configured to receive the radio-frequency power applied from the radio-frequency power applying unit; a gas dispersion plate formed of a conductive material, the gas dispersion plate having a recess formed near a center portion, the gas dispersion plate being in closely contact with the antenna electrode unit near a periphery portion, the gas dispersion plate having a space formed between the gas dispersion plate and the antenna electrode unit, the gas dispersion plate storing the processing gas supplied from the gas supplying unit, into the space; and a shower plate formed of an insulating member, the shower plate covering the gas dispersion plate, the shower plate having a large number of holes formed for supplying the processing gas stored in the space formed between the antenna electrode unit and the gas dispersion plate, into the processing room, the shower plate having an annular groove formed on a side facing the gas dispersion plate, a conductive member being embedded in the annular groove, the conductive member electrically connecting with the gas dispersion plate. 
     According to the present invention, plasma having excessively high uniformity of electron density from a center portion to a circumferential portion of an electrode, can be generated, so that an etching rate distribution having high uniformity in a surface of a wafer can be achieved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic longitudinal sectional view of the schematic configuration of a plasma processing apparatus according to Example of the present invention; 
         FIGS. 2A and 2B  each are an enlarged schematic longitudinal sectional view of the schematic configuration of an antenna unit and the periphery thereof in the plasma processing apparatus according to the present Example illustrated in  FIG. 1 ; 
         FIGS. 3A to 3D  are schematic lower views of modifications of the configuration of the antenna unit according to the present Example illustrated in  FIGS. 2A and 2B ; 
         FIG. 4  is a graph exemplifying an etching rate in etching processing to a semiconductor wafer by the plasma processing apparatus according to Example illustrated in  FIG. 1 ; 
         FIG. 5  is a graph exemplifying the variation in the position of a region in which electron density drops in the radial direction of the wafer, to the variation in the frequency of plasma formation radio-frequency power, in the plasma processing apparatus according to Example illustrated in  FIG. 1 ; 
         FIG. 6A  is a graph exemplifying the distribution of the electron density of plasma in the radial direction of a wafer according to a conventional technique, and  FIG. 6B  is a graph exemplifying the distributions of the electron density of plasma in the radial direction of a wafer in a plurality of cases where a protrusion is disposed at different positions in the radial direction of the wafer in the plasma processing apparatus according to Example illustrated in  FIG. 1 ; 
         FIG. 7  is a graph of the relationship between the variation in the ratio between the height of the protrusion and the thickness of a shower plate in the plasma processing apparatus according to Example illustrated in  FIG. 1  and the etching rate in the etching processing to the wafer by the plasma processing apparatus; and 
         FIG. 8  is a graph of the relationship between the ratio between the width of a recess and the diameter of the shower plate in the plasma processing apparatus illustrated in  FIG. 1  and the variation in the etching rate in the etching processing of the plasma processing apparatus. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of the present invention will be described below with reference to the drawings. 
     EXAMPLE 
     Example 1 according to the present invention will be described with  FIG. 1  and  FIGS. 2A and 2B .  FIG. 1  is a schematic longitudinal sectional view of the schematic configuration of a plasma processing apparatus according to Example of the present invention. 
     The plasma processing apparatus  100  according to the present Example, is a plasma etching apparatus including: a processing room in which plasma is to be formed with internal decompression; and discoid electrodes to be supplied with radio-frequency power, disposed above and below through a space in which the plasma is to be formed in the processing room. The plasma etching apparatus performs, with the plasma, etching processing to a substrate-shaped sample, such as a semiconductor wafer, disposed on a sample stage disposed inside the processing room, the sample stage including the lower electrode from the upper and lower electrodes, built in. Particularly, the plasma processing apparatus  100  is a parallel-plate-type plasma processing apparatus in which an electric field due to the supplied radio-frequency power is introduced from the surface of the upper electrode into the processing room and additionally a magnetic field formed by coils disposed surrounding the upper and lateral periphery of the processing room outside a vacuum chamber is supplied to the processing room, and then the radio-frequency power and the plasma formed by excitation and dissociation of the atoms or molecules of gas introduced into the processing room are capacitively coupled. 
     In the configuration illustrated in  FIG. 1 , the plasma processing apparatus  100  includes a vacuum chamber  125  having a cylindrical shape, the vacuum chamber  125  internally including a processing room  101  having a space having a cylindrical shape. The plasma processing apparatus  100  includes: an upper electrode  10  and a lower electrode  12  disposed above and below inside the vacuum chamber  125  through a space in which plasma  111  is to be formed in the processing room  101 ; and a radio-frequency power source  112  and a bias formation radio-frequency power source  116  electrically connected to the upper electrode  10  and the lower electrode  12 , respectively, the radio-frequency power source  112  and the bias formation radio-frequency power source  116  being configured to supply radio-frequency power having a predetermined frequency to the upper electrode  10  and the lower electrode  12 , respectively. 
     The vacuum chamber  125  includes a vacuum exhaust unit  1200  including an exhaust pump  120 , such as a turbo-molecular pump, that exhausts and decompresses the particles of gas and the plasma  111  inside the processing room  101  in communication with the processing room  101 . An exhaust opening  1202  facing the processing room  101 , of an exhaust duct  1201  forming an exhaust channel disposed between the inlet of the exhaust pump  120  and the processing room  101 , is disposed below the upper surface of the lower electrode  12 . 
     The lower electrode  12  includes: a stage (electrode body)  102  formed of a metal member, being a sample stage disposed below the space in which the plasma  111  is to be formed in the processing room  101 ; an insulating member  1020  electrically insulating the stage  102  from the vacuum chamber  125 , provided between the stage  102  and a wall surface of the vacuum chamber  125 ; and a dielectric film  121  on which a wafer  103  is to be mounted, formed on the stage  102 . The lower electrode  12  is disposed, opposed to the upper electrode  10  disposed above. 
     An antenna unit included in the upper electrode  10 , is disposed above the lower electrode  12 , opposed to the lower electrode  12 . The antenna unit (upper electrode  10 ) according to the present Example, includes: a conductive antenna body  107  having a discoid shape; a gas dispersion plate  108 ; and a shower plate  110 . 
     The conductive antenna body  107  having the discoid shape, is electrically connected to the radio-frequency power source  112  that supplies the radio-frequency power in a VHF band, through a waveguide, such as a coaxial cable  205 . 
     The gas dispersion plate  108  is disposed below the antenna body  107 , and includes a member having a discoid or cylindrical shape. A gas supply source  109  introduces processing gas into the gas dispersion plate  108 , and then the gas dispersion plate  108  internally disperses the processing gas. 
     The shower plate  110  is disposed below the gas dispersion plate  108 , and is included in the ceiling surface of the processing room  101 . The shower plate  110  has gas introducing holes being a plurality of through holes through which the processing gas that has been dispersed, passes to be introduced into the processing room  101 . A conductive protrusion  202  is embedded, in a ring shape, into a groove formed on the shower plate  110 , and the upper surface of the conductive protrusion  202  is in contact with the gas dispersion plate  108 . 
     The antenna unit (upper electrode  10 ) is disposed inside an upper lid member  1251  of the vacuum chamber  125 , through a ring-shaped insulating ring  122  including a dielectric member, such as quartz, for insulation. 
     The insulating ring  122  on the circumferential side of the antenna unit (upper electrode  10 ) surrounds, in a ring shape, the periphery of the antenna unit (upper electrode  10 ) between the antenna unit (upper electrode  10 ) and the lid member  1251 . The lower end surface of the circumferential portion of the insulating ring  122  is disposed at a height position (so-called surface position) the same as or approximate to the height position of the lower surface of the shower plate  110 , surrounding the circumference of the shower plate  110 , and is included in the ceiling surface of the processing room  101 . 
     According to the present Example, the antenna body  107 , the gas dispersion plate  108 , and the ring-shaped protrusion  202  included in the upper electrode  10 , each are formed of a conductive material, such as aluminum, and the shower plate  110  facing the space in which the plasma  111  is to be formed in the processing room  101 , is formed of a dielectric material, such as quartz. 
     The antenna body  107  is electrically connected to the radio-frequency power source  112  that supplies the radio-frequency power in the VHF band for generating the plasma  111 , with the coaxial cable  205  through a first matching device  113 . The antenna body  107  is connected to a location at ground potential through a filter  114  such that the antenna body  107  functions as a ground electrode for the radio-frequency power supplied to the lower electrode  12 , together with the gas dispersion plate  108 . 
     The filter  114  is designed to block the power for the plasma generation in the VHF band, applied to the antenna body  107  of the antenna unit (upper electrode  10 ) by the radio-frequency power source  112  and to pass the radio-frequency power for forming a bias potential above the upper surface of the wafer  103 , supplied to the stage  102  on which the wafer  103  is mounted, included in the lower electrode  12 . 
     In order to reduce damage to the internal wall of the processing room  101 , at the potential of the plasma  111  reduced, with inhibition of the plasma  111  from being excessively dissociated, at the electron density of the plasma  111  set up to approximately 10 10  cm −3 , the frequency of the radio-frequency power generated by the radio-frequency power source  112  is desirably in a range of 50 to 500 MHz, and thus a frequency of 200 MHz is used according to the present Example. The radio-frequency power having the frequency of 200 MHz, supplied from the radio-frequency power source  112  to the antenna unit (upper electrode  10 ) through the coaxial cable  205 , is supplied to the antenna body  107  and the conductive gas dispersion plate  108  connected to the antenna body  107 . Then, the radio-frequency power is emitted from the surface on the side of the shower plate  110  of the gas dispersion plate  108 , into the processing room  101  through the shower plate  110 . 
     A first coil  104  and a second coil  105  are disposed surrounding the vacuum chamber  125 , the internal antenna unit (upper electrode  10 ), and the coaxial cable  205 , in a ring shape, on the upper and lateral periphery of a cylindrical portion of the processing room  101 , outside the vacuum chamber  125 . 
     A direct current supplied from a power source not illustrated, to the first coil  104  and the second coil  105  causes a magnetic field capable of improving the efficiency of heating the plasma  111  occurring inside the processing room  101  due to the radio-frequency power having the frequency of 200 MHz supplied from the radio-frequency power source  112 . A conductive yoke  106  disposed covering the circumferential and upper sides of the first coil  104  and the second coil  105 , adjusts the magnetic field generated by the first coil  104  and the second coil  105  such that the magnetic field has a distribution in which, when viewed from above the central axis in the upper and lower direction of the antenna unit (upper electrode  10 ) and the processing room  101 , the lines of magnetic force head radially around the central axis, in  FIG. 1 , head downward to and outward from the processing room  101  (in the left and right direction in  FIG. 1 ), namely, head downward along the central axis and gradually spread. 
     By, for example, a thermal spraying method, the dielectric film  121  formed of a dielectric material including ceramics, such as alumina or yttria, is disposed covering the upper surface of the stage  102  included in the lower electrode  12  disposed below the processing room  101 . The dielectric film  121  forms the mount surface of the lower electrode  12  on which the wafer  103  is mounted. 
     A plurality of electrostatic adsorption electrodes  123  and  124  is disposed inside the dielectric film  121 , in order for the wafer  103  to adsorb and remain onto the dielectric film  121  with electrostatic force formed by supply of direct current power with the wafer  103  mounted on the dielectric film  121 . The electrostatic adsorption electrode  123  is connected to a first direct current power source  117 , and the electrostatic adsorption electrode  124  is connected to a second direct current power source  118 . 
     A refrigerant channel (not illustrated) disposed multiply coaxially or spirally around the center of the stage  102  having a cylindrical shape, is disposed inside the stage  102  included in the lower electrode  12  through the insulating member  1020 , the refrigerant channel being coupled to a temperature controller, such as a chiller unit, not illustrated, through a pipe. A refrigerant, such as a coolant, adjusted at a temperature in a predetermined range by the temperature controller, flows into the refrigerant channel through the pipe not illustrated and passes through and flows out of the refrigerant channel. Then, the refrigerant circularly returns to the temperature controller. This arrangement retains the temperature of the stage  102 , furthermore, the temperature of the wafer  103  electrostatically adsorbing on the dielectric film  121  on the upper surface of the stage  102 , at a value in a range appropriate to the processing. 
     Furthermore, the stage  102  and the insulating member  1020  have a path  1021  formed penetrating inside, the path  1021  having an upper end opening disposed at the upper surface of the dielectric film  121 , the lower end of the path  1021  being coupled to a heat exchange gas supply source  119 . 
     With the wafer  103  electrostatically adsorbing and remaining on the upper surface of the dielectric film  121  due to the electrostatic adsorption electrode  123  connected to the first direct current power source  117  and the electrostatic adsorption electrode  124  connected to the second direct current power source  118 , the heat exchange gas supply source  119  supplies a heat exchange gas, such as He, to the gap between the upper surface of the dielectric film  121  and the back surface of the wafer  103 , through the path  1021 . Then, heat transfer increases between the two and heat exchange accelerates between the wafer  103  and the stage  102 , so that the responsiveness and precision of adjustment of the temperature of the wafer  103  due to the heat exchange with the stage  102 , improve. 
     The exhaust opening  1202  for exhausting the particles of the gas, the plasma, or a reaction product inside the processing room  101 , is disposed on the wall surface of the processing room  101  below the upper surface of the stage  102 , the exhaust opening  1202  being coupled to the exhaust pump  120  being a vacuum pump included in the vacuum exhaust unit  1200 , through the exhaust duct  1201 . An exhaust adjustment valve not illustrated that increases or decreases the sectional area of an exhaust path inside the duct to increase or decrease the flow or speed of exhaust, is disposed on the exhaust duct  1201  between the inlet of the exhaust pump  120  and the exhaust opening  1202 . 
     With the configuration described above, first, with the wafer  103  mounted on the upper surface of the dielectric film  121  of the lower electrode  12  by a conveying unit not illustrated, the first direct current power source  117  applies the direct current power to the electrostatic adsorption electrode  123  and the second direct current power source  118  applies the direct current power to the electrostatic adsorption electrode  124 , to generate the electrostatic force on the upper surface of the dielectric film  121 , so that the wafer  103  electrostatically adsorbs onto the upper surface of the dielectric film  121 . 
     With the wafer  103  adsorbing and remaining on the upper surface of the dielectric film  121  due to the electrostatic force, the processing gas is introduced from the plurality of gas introducing holes  214  (refer to FIGS.  2 A and  2 B) formed through the shower plate  110  of the antenna unit (upper electrode  10 ), into the processing room  101 , and additionally the exhaust pump  120  of the vacuum exhaust unit  1200  operates to exhaust inside the processing room  101 . 
     At this time, the exhaust adjustment valve not illustrated provided in the vacuum exhaust unit  1200 , adjusts the degree of opening to balance the flow or speed of exhaust with the flow or speed of the gas supplied inside the processing room  101  by a gas flow controller (mass flow controller) not illustrated disposed inside the gas supply source  109  or on a gas supply path  1091  between the gas supply source  109  and the gas dispersion plate  108 , so that pressure inside the processing room  101  can be adjusted to a value in a range appropriate to the processing of the wafer  103 . 
     In this manner, with the pressure inside the processing room  101  adjusted to the value in the range appropriate to the processing of the wafer  103 , the radio-frequency power source  112  applies the radio-frequency power in the VHF band, to the antenna body  107  of the upper electrode  10  through the first matching device  113 , and the direct current power source not illustrated applies the direct current to the first coil  104  and the second coil  105 . As a result, an electric field is formed from the lower surface (side of the shower plate  110 ) of the gas dispersion plate  108  of the antenna unit (upper electrode  10 ) to the shower plate  110 , and the magnetic field generated by the first coil  104 , the second coil  105 , and the yoke  106  is formed inside the processing room  101 . 
     This arrangement excites and dissociates the gas introduced from the plurality of gas introducing holes  214  of the shower plate  110  into the processing room  101 , so that the plasma  111  occurs in the space of the processing room  101  between the upper electrode  10  and the lower electrode  12 . 
     The stage  102  formed of the metal member, in the lower electrode  12  is electrically connected to the bias formation radio-frequency power source  116  through a second matching device  115 . With the plasma  111  formed, the bias formation radio-frequency power source  116  applies the bias formation radio-frequency power having the predetermined frequency, to the stage  102 , so that the bias potential is formed above the wafer  103  electrostatically adsorbing on the dielectric film  121  formed on the upper surface of the stage  102 . With this condition retained, energy corresponding to the potential difference between the potential of the plasma  111  and the bias potential, accelerates the charged particles, such as ions, in the plasma  111 , so that the charged particles are attracted to the wafer  103  to collide with the wafer  103 . This arrangement performs etching processing to the surface of a film layer to be processed included in a film structure previously formed on the upper surface of the wafer  103 . 
     The frequency of the bias formation radio-frequency power applied to the stage  102  by the bias formation radio-frequency power source  116  according to the present Example, is desirably in a range of 400 kHz to 4 MHz sufficiently lower than the frequency of 200 MHz of the radio-frequency power applied to the antenna body  107  by the radio-frequency power source  112 , in order not to exert influence on the density distribution of the charged particles or the intensity distribution in the plasma  111 . The generation of the plasma  111  due to the bias formation radio-frequency power supplied by the bias formation radio-frequency power source  116 , can be negligibly reduced as long as in the frequency range of 400 kHz to 4 MHz. 
     Meanwhile, the variation width of the energy of the charged particles, such as ions, attracted to the wafer  103 , narrows as the frequency of the bias formation radio-frequency power supplied by the bias formation radio-frequency power source  116  rises. Thus, control of the collision energy of the ions, can improve controllability such as adjustment of a processing characteristic, for example, the speed of the etching processing. According to the present Example, the frequency of the bias formation radio-frequency power applied to the stage  102  by the bias formation radio-frequency power source  116 , was set to 4 MHz. 
     The detailed configuration of the antenna unit (upper electrode  10 ) according to the present Example, will be described with  FIGS. 2A and 2B  and  FIGS. 3A to 3D .  FIGS. 2A and 2B  each are an enlarged schematic longitudinal sectional view of the schematic configuration of the antenna (upper electrode  10 ) and the periphery thereof in the plasma processing apparatus  100  according to the present Example illustrated in  FIG. 1 .  FIGS. 3A to 3D  are schematic plan views of modifications of the configuration of the antenna unit (upper electrode  10 ) illustrated in  FIGS. 2A and 2B  when viewed from the side of the lower electrode  12 . 
     In the example illustrated in  FIG. 2A , the antenna unit (upper electrode  10 ) has a center portion on the upper surface of the conductive antenna body  107  having the discoid shape, connected with the coaxial cable  205 , and the radio-frequency power source  112  supplies the radio-frequency power to the antenna body  107  through the coaxial cable  205 . The conductive gas dispersion plate  108  having the discoid shape having a diameter the same as that of the antenna body  107 , below the antenna body  107  (side of the lower electrode  12 ), is connected to the antenna body  107  such that the vicinity of the circumferential portion of the gas dispersion plate  108  is in contact with the antenna body  107 . 
     Furthermore, the dielectric shower plate  110  having a discoid or cylindrical shape, below the gas dispersion plate  108  (side of the lower electrode  12 ), is coupled to the gas dispersion plate  108 , the upper surface of the shower plate  110  covering the lower surface of the gas dispersion plate  108 , the upper surface being opposed to the lower surface. 
     A sealing groove  1081  is formed on the lower surface of the gas dispersion plate  108 , namely, on the side facing the shower plate  110 , along the circumference. The gas dispersion plate  108  and the shower plate  110  come in contact with each other with a sealing member  1082 , such as an O ring, attached to the sealing groove  1081 , the sealing member  1082  being sandwiched between the shower plate  110  and the sealing groove  1081 . As a result, the inside and outside of the sealing member  1082  are hermetically sealed. 
     A sealing groove  1071  having a predetermined sectional shape, is formed in the vicinity of the circumferential portion of the lower surface of the antenna body  107 , namely, on a portion in contact with the upper surface of the gas dispersion plate  108 , along the circumferential portion of the lower surface of the antenna body  107 . The antenna body  107  and the gas dispersion plate  108  come in contact with each other with a sealing member  1072 , such as an O ring, attached to the sealing groove  1071 , the sealing member  1072  being sandwiched between the sealing groove  1071  and the gas dispersion plate  108 . As a result, the inside and outside of the sealing member  1072  are hermetically sealed. 
     Here, the gas dispersion plate  108  has a recess  1083  formed in an internal portion a width away from the cylindrical circumferential surface thereof, along the circumferential surface. The contact between the antenna body  107  and the gas dispersion plate  108  with the sealing member  1072 , such as the O ring, attached to the sealing groove  1071 , allows the recess  1083  to form a buffer room  201  between the gas dispersion plate  108  and the antenna body  107 . 
     The buffer room  201  is coupled to the gas supply source  109  through the gas supply path  1091 , communicating with the gas supply source  109 . The gas supply source  109  introduces the gas into the buffer room  201 , so that the gas diffuses inside. The gas dispersion plate  108  forming the lower surface of the buffer room  201  and the shower plate  110  disposed below the gas dispersion plate  108 , have a plurality of gas introducing holes  204  and the plurality of gas introducing holes  214  penetrating therethrough, respectively, each having a diameter of approximately 0.3 to 1.5 mm, each being fine. The gas supplied from the gas supply source  109 , diffusing in the buffer room  201 , is introduced into the processing room  101  below through the gas introducing holes  204  formed through the gas dispersion plate  108  and the gas introducing holes  214  formed through the shower plate  110 . 
     According to the present Example, furthermore, the recess  203  is formed in a ring shape around the central axis of the shower plate  110 , on the surface in contact with the gas dispersion plate  108 , of the shower plate  110 . The conductive protrusion  202  formed in the ring shape is embedded into the recess  203 . The conductive protrusion  202  has a thickness set in consideration of the depth of the recess  203  such that the upper surface of the conductive protrusion  202  is in contact with the gas dispersion plate  108  with the conductive protrusion  202  embedded in the recess  203 . That is, the thickness of the plate-like shower plate  110  is reduced by the depth of the recess  203 , at the portion having the recess  203  formed, of the shower plate  110 . 
     With the upper surface of the shower plate  110  and the lower surface of the gas dispersion plate  108  coupled opposed to each other, the conductive protrusion  202  is embedded inside the recess  203  and thus the recess  203  is internally filled with the conductive material of the protrusion  202 . The distance from the bottom surface (side of the lower electrode  12 ) of the protrusion  202  in contact with the gas dispersion plate  108 , to the bottom surface (side of the lower electrode  12 ) of the shower plate  110 , is smaller than the distance between the bottom surface (side of the lower electrode  12 ) of the shower plate  110  at a location different from the recess  203  and the bottom surface (side of the lower electrode  12 ) of the gas dispersion plate  108 . 
     According to the present Example, the position of the ring-shaped protrusion  202  embedded in the recess  203  formed on the shower plate  110 , is arranged such that the circumferential portion of the ring-shaped protrusion  202  is in a region inside the circumferential edge of the wafer  103  when the wafer  103  mounted on the lower electrode  12  is viewed from the side of the upper electrode  10 . That is, the circumferential edge of the ring-shaped protrusion  202  coaxially disposed around the axis passing through the center of the wafer  103  in the upper and lower direction, is disposed at a position smaller than the diameter of the wafer  103 . 
     Particularly according to the present Example, the wafer  103  is approximately 300 mm in diameter, and the circumferential edge of the ring-shaped protrusion  202  is disposed at a position in a range of 50 to 100 mm in the radial direction from the center of the gas dispersion plate  108  coaxially disposed. Furthermore, the thickness of the protrusion  202  (height of the protrusion  202 ) is set to a value of 1 to 5 mm and the size in the radial direction (width of the ring of the protrusion  202  formed in the ring shape) is set to a value of 5 to 30 mm. Particularly, according to the present Example, the position of the central point (an intermediate location between the inner radius and outer radius of the protrusion  202 ) in the width in the radial direction of the protrusion  202  from the center of the gas dispersion plate  108 , was 80 mm and the height and width of the protrusion  202  were 4 and 20 mm, respectively. 
     With the protrusion  202  inserted into the recess  203  formed on the shower plate  110  and the gas dispersion plate  108  attached to the shower plate  110 , the protrusion  202  formed of a conductor, such as metal, is electrically connected to the gas dispersion plate  108  in contact with the gas dispersion plate  108 . With this condition retained, when the radio-frequency power source  112  applies the radio-frequency power to the antenna body  107 , the radio-frequency power is also supplied to the protrusion  202  through the gas dispersion plate  108 . Note that, a gas introducing hole  2024  is formed inside the protrusion  202 , penetrating through the protrusion  202 , the gas introducing hole  2024  being connected to a gas introducing hole  204  formed through the gas dispersion plate  108  and a gas introducing hole  214  formed through the shower plate  110 . 
       FIG. 2B  illustrates a modification of the protrusion  202  formed of the conductor, such as metal, in the antenna unit (upper electrode  10 ) illustrated in  FIG. 2A . A protrusion  2021  formed of a conductor, such as metal, in an antenna unit (upper electrode  10 - 1 ) illustrated in  FIG. 2B , has a recess  2022  formed on the side facing the gas dispersion plate  108 . With the protrusion  2021  connected to the lower surface of the gas dispersion plate  108 , abutting on the lower surface of the gas dispersion plate  108 , a gap is formed due to the recess  2022  between the protrusion  2021  and the gas dispersion plate  108 . 
     This configuration allows a gas introducing hole  204  formed through the gas dispersion plate  108  to directly communicate with the gap due to the recess  2022  and a gas introducing hole  214  formed through the shower plate  110  to communicate with the gap due to the recess  2022  through a gas introducing hole  20214  formed through the protrusion  2021 . This configuration introduces the gas supplied to the buffer room  201 , into the processing room  101  at a portion of the protrusion  2021  through the gas introducing hole  204  and the gap due to the recess  2022 . Note that, the lower surface (side in contact with the shower plate  110 ) and side wall surface of the protrusion  2021  in the figure, abut on the inner wall surface and bottom of the recess  203  in which the protrusion  2021  is to be embedded, disposed at a corresponding position on the back surface of the shower plate  110 , such that a gap between the protrusion  2021  and the recess  203  is as small as possible. 
       FIG. 3A  is a plan view of the schematic configuration of the gas dispersion plate  108  and the protrusion  202  formed of the conductor, such as metal, disposed below the gas dispersion plate  108 , in the antenna unit (upper electrode  10 ) illustrated in  FIG. 2A  when viewed from below (side of the lower electrode  12 ). As illustrated in the present figure, the protrusion  202  is a ring-shaped member coaxially disposed around the center of the gas dispersion plate  108 . Note that, the protrusion  202  may include a plurality of members instead of a solid member as illustrated in  FIG. 3A . In addition, the protrusion  202  may be radially disposed at a plurality of positions, namely, multiply disposed, instead of at a single radial position. 
       FIG. 3B  exemplifies a protrusion  202 - 1  including a plurality of arc conductive members circumferentially disposed in a ring shape at radially the same positions from the center when viewed from below, according to a modification of Example illustrated in  FIG. 3A .  FIG. 3C  exemplifies protrusions  202 - 2  and  202 - 3  including two conductive ring-shaped members at radially a plurality of positions, namely, at different radial positions, circumferentially integrally formed, when viewed from below.  FIG. 3D  exemplifies a plurality of conductive members  202 - 4  each having a cylindrical shape, disposed in a ring shape at radially the same positions around the center. 
       FIG. 4  illustrates the comparison between the distribution of etching speed (etching rate)  401  in the semiconductor wafer  103  subjected to the etching processing by the plasma processing apparatus  100  according to the present Example and the distribution of etching speed (etching rate)  402  in the etching processing performed by a conventional technique in which the antenna unit (upper electrode  10 ) includes no conductive protrusion  202  (conventional example). 
     The distribution of the etching rate  401  in the graph illustrated in  FIG. 4 , exemplifies the in-wafer-plain distribution of the etching rate in the semiconductor wafer  103  subjected to the etching processing by the plasma processing apparatus  100  according to the present Example illustrated in  FIG. 1 . The horizontal axis represents the distance from the wafer center, and the vertical axis represents the relative value of the etching rate. 
     The distribution from the wafer center in the distribution of the etching rate  402  illustrated as the conventional example in the graph of  FIG. 4 , is a result of the etching processing with an etching apparatus including the antenna unit having a configuration different from the configuration of the antenna unit (upper electrode  10 ) illustrated in  FIG. 2A  according to the present Example. That is, the etching apparatus performing the etching processing for the distribution of the etching rate  402  illustrated as the conventional example in the graph of  FIG. 4 , includes no protrusion  202  and no recess  203  in which the protrusion  202  is to be embedded, disposed between the gas dispersion plate  108  and the shower plate  110  described in the present Example. The gas dispersion plate  108  and the shower plate  110  are coupled to each other, the flat lower surface of the gas dispersion plate  108  and the flat upper surface of the shower plate  110  being opposed to each other. Particularly,  FIG. 4  exemplifies the result of the etching processing to a resist for photolithography performed by each of the plasma processing apparatus according to the present Example and the etching apparatus according to the conventional technique (conventional example). 
     The etching processing was performed to a silicon wafer having a diameter of 300 mm coated with the resist for photolithography, with the plasma formed with mixed gas of SF6 and CHF3 as the processing gas in a condition of a pressure of 4 Pa in the processing room, a plasma formation radio-frequency power of 800 W, a frequency of 200 MHz, and a bias formation radio-frequency power of 50 W above the upper surface of the wafer. 
     For the etching processing performed by the conventional plasma processing apparatus including no conductive protrusion between the gas dispersion plate and the shower plate (in comparison to the configuration of the plasma processing apparatus  100  according to the present Example illustrated in  FIG. 1 , no conductive protrusion  202  is present and the shower plate  110  has no groove to which the conductive protrusion  202  is to be embedded. The opposed surfaces of the gas dispersion plate  108  and the shower plate  110 , are entirely in contact with each other.), as illustrated in  FIG. 4 , a drop in the etching rate was observed in a region at a radial position of 50 to 100 mm on the wafer in the distribution of the etching rate  402  illustrated as the conventional example. 
     In contrast to this, the drop in the etching rate was dramatically improved and the variation in the etching rate was reduced in the in-plane radial direction on the upper surface of the wafer, in the distribution of the etching rate  401  in the processing of the plasma processing apparatus  100  according to the present Example. 
     The frequency of the plasma formation radio-frequency power of the etching apparatus in the conventional example illustrated in  FIG. 4 , was set to 200 MHz the same as that according to the present Example. 
     It can be considered that a reason for the occurrence of the drop in the etching rate in the region at the radial position of 50 to 100 mm from the center of the wafer  103  in the distribution of the etching rate  402  illustrated as the conventional example of  FIG. 4 , is as follows. That is, the intensity distribution of the electric field formed in the processing room by the power having the frequency supplied to the antenna unit, furthermore, the intensity distribution or density distribution of the plasma formed with the electric field, are expressed with the superposition of Bessel functions. As a result, a distribution having a high value at the center portion of the processing room, is acquired. In accordance with the distribution, the electron density of the plasma formed in the processing room by only the electric field, has also a high value at the center portion. 
     For the etching apparatus that forms the distribution of the electric field, used as the conventional example, a magnetic field forming unit, such as a coil, is provided outside the processing room, to form a magnetic field inside the processing room. Then, the magnetic field is adjusted to improve power absorption efficiency toward the circumferential side of the wafer, so that the electron density can be uniformed to some extent. 
     In the etching apparatus used as the conventional example described above, the formation of the downward and outward spread magnetic field in the processing room  101  by the first coil  104 , the second coil  105 , and the yoke  106  disposed coaxially surrounding the processing room around the central axis thereof on the upper and lateral outside of the processing room  101 , causes the distribution of the electron density in the processing room  101 , to horizontally improve from the center to the outside, to correct the convex distribution of the electric field, so that the electron density in the plasma  111  can further functionally approximate to uniformity. 
     However, it is technically difficult to cause the gradient of the electric field to agree with the gradient of the magnetic field in the radial direction of the upper electrode  10  and the lower electrode  12  above the lower electrode  12 . Thus, a region in which the electron density locally decreases, is formed between the center and circumferential edge of the discoid members of the antenna unit being the upper electrode  10  supplied with the radio-frequency power. The local drop in the electron density, is a factor in the drop in the etching rate at the position in the radial direction of the wafer  103 , corresponding to the location, so that the uniformity of the in-wafer-plane etching rate degrades. 
     Meanwhile, for the distribution of the etching rate  401  illustrated as the present Example, the recess  203  is formed at a coaxial position to the antenna body, on the shower plate  110  attached to the lower surface of the gas dispersion plate  108  electrically connected to the antenna body  107 . The conductive protrusion  202  is embedded in the recess  203 . The depth of the recess  203  and the height (thickness) of the protrusion  202  are set such that the conductive protrusion  202  is electrically connected to the gas dispersion plate  108  in contact with the gas dispersion plate  108  when the shower plate  110  is combined with the gas dispersion plate  108  with the conductive protrusion  202  embedded in the recess  203 . 
     In this manner, the contact between the gas dispersion plate  108  and the protrusion  202 , causes the dielectric shower plate  110  to locally radially increase or decrease in thickness due to the protrusion  202 . 
     Assuming that the dielectric shower plate  110  is a waveguide for electromagnetic waves, a rapid variation in the height of the shower plate  110  corresponding to the waveguide causes susceptance, so that the intensity of the electric field increases, at the recess  203 , vertically to the antenna body  107  or the gas dispersion plate  108 . In accordance with a radially and locally ring-shaped increase in the intensity of the electric field, the electron density increases in the plasma  111  at a location directly below the protrusion  202  and a region in proximity to the location, above the lower electrode  12  in the processing room  101 . As a result, the variation in the etching rate radially decreases in the surface of the wafer  103 , so that the uniformity of the etching rate can improve. 
     According to the present Example, it is important that the conductive protrusion  202  is disposed at a position corresponding to a region in which a drop easily occurs in the electron density of the plasma  111  above the wafer  103  mounted on the lower electrode  12 . Meanwhile, the position of a region in which the electron density easily drops in the radial direction of the wafer  103  mounted on the lower electrode  12 , varies depending on the frequency for generating the plasma  111 . 
       FIG. 5  illustrates an exemplary relationship between the position at which the electron density locally drops on the wafer  103 , namely, the position in the radial direction from the center of the wafer  103  mounted on the lower electrode  12  or the center of the upper electrode  10 , and the frequency of the radio-frequency power applied from the radio-frequency power source  112  to the upper electrode  10 , in the plasma processing apparatus  100  according to the present Example. An exemplary distribution of the electron density when the frequency of the radio-frequency power applied from the radio-frequency power source  112  to the upper electrode  10  in order to generate the plasma  111 , varied, will be described with  FIGS. 6A and 6B . 
       FIG. 5  is a graph in which a curve  501  indicates an exemplary variation in the position of the region in which the electron density drops in the radial direction of the wafer  103  mounted on the lower electrode  12 , to the variation in the frequency of the plasma formation radio-frequency power supplied from the radio-frequency power source  112  to the upper electrode  10 , in the plasma processing apparatus  100  according to the present Example illustrated in  FIG. 1 . 
     As indicated with the curve  501  of  FIG. 5 , the distribution of the electron density (occurrence position of the region in which the electron density drops in the radial direction of the wafer  103 ) varies in accordance with the frequency of the plasma formation radio-frequency power applied from the radio-frequency power source  112  to the upper electrode  10 . That is, it can be seen that the region in which the electron density locally drops, approximates to the circumferential edge of the wafer  103  as the frequency of the plasma formation radio-frequency power decreases. 
     From  FIG. 5 , it can be seen that the region in which the electron density locally drops, is formed at a position of approximately 80 mm in the radial direction from the center of the wafer  103  at the frequency of 200 MHz of the plasma formation radio-frequency power used in the present Example. According to the present Example, the protrusion  202  is disposed such that the center of the width of the protrusion  202  is positioned at a position corresponding to this position, specifically, at a position of 80 mm in the radial direction from the center of the gas dispersion plate  108 . 
       FIG. 6A  is a graph exemplifying the distribution of the electron density of the plasma  601  in the radial direction of the wafer mounted on the lower electrode  12 , in the plasma processing apparatus used as the conventional example including: no conductive protrusion  202 ; no groove in which the conductive protrusion  202  is to be embedded, formed on the shower plate  110 ; and the opposed surfaces of the gas dispersion plate  108  and the shower plate  110  entirely being in contact with each other, in comparison to the configuration according to the present Example described in  FIG. 1 , as described in  FIG. 4 . 
       FIG. 6B  is a graph exemplifying the distribution of the electron density of the plasma  602  in the radial direction of the wafer in a plurality of cases where the conductive protrusion  202  was disposed at different positions in the radial direction of the wafer in the plasma processing apparatus  100  according to the present Example illustrated in  FIG. 1 . 
       FIG. 6B  illustrates results of the distribution of the electron density of the plasma  603  for the protrusion  202  disposed having the center in width at a position of 60 mm in the radial direction of the wafer  103  as Comparative Example 1 and the distribution of the electron density of the plasma  604  for the protrusion  202  disposed having the center in width at a position of 100 mm in the radial direction of the wafer  103  as Comparative Example 2, in comparison to the present Example for the protrusion  202  disposed having the center in width at the position of 80 mm in the radial direction. 
     The variation is reduced in the value of the electron density in the radial direction, in the distribution of the electron density of the plasma  602  according to the present Example in which the protrusion  202  was disposed at the position of 80 mm in the radial direction of the wafer  103 , illustrated in  FIG. 6B , in comparison to the distribution of the electron density of the plasma  601  according to the conventional example illustrated in  FIG. 6A  in which the region in which the electron density locally drops in the radial direction of the wafer  103 , is present with no countermeasure against the local drop of the electron density in the radial direction of the wafer  103 . 
     Meanwhile, in the distributions of the electron density of the plasma  603  and  604  according to Comparative Examples 1 and 2 in which the protrusion  202  was disposed at 60 and 100 mm in the radial direction illustrated in  FIG. 6B , respectively, the region in which the electron density locally drops, moves in the radial direction in comparison to the conventional example. However, since the degree of improvement for the drop of the electron density may be small, the difference between a maximum value and a minimum value formed, is larger than the local drop in the distribution of the electron density of the plasma  601  in the conventional example illustrated in  FIG. 6A . 
     As described above, it can be seen that an appropriate positional range is present for the disposition of the conductive protrusion  202  electrically integrated with the gas dispersion plate  108  in contact with the gas dispersion plate  108 , in order to effectively reduce the variation in the electron density in the radial direction of the wafer  103 . It can be seen that the disposition of the conductive protrusion  202  in the range is important in order to improve the uniformity of the plasma processing in the surface of the wafer  103  so that the yield of the plasma processing improve. 
     Next, the relationship between the height of the protrusion  202  and the variation in the etching rate, will be described with  FIG. 7 .  FIG. 7  is a graph of the relationship between the variation in the ratio between the height (thickness) of the conductive protrusion  202  and the thickness of the shower plate  110  in the plasma processing apparatus  100  according to the present Example illustrated in  FIG. 1  and the variation in the etching rate  701  in the etching processing to the wafer  103  by the plasma processing apparatus  100 . 
     In the present figure, the height (thickness) of the conductive protrusion  202  and the depth of the recess  203  of the shower plate  110  each are defined as d, and the thickness of the shower plate  110  is defined as t. According to the present Example, the thickness t of the shower plate  110  is 16 mm. The relationship between the thickness t of the shower plate  110  and the depth d of the recess  203  is defined as d/t. Illustrated is the root-mean-square value (variation) in deviation between the average value of values in the etching rate at positions in the radial direction from the center to the circumferential edge of the wafer  103  acquired in the etching processing to the wafer  103  with the variation in d/t and the values in the etching rate at the positions. 
     As illustrated in  FIG. 7 , the variation in the etching rate  701  decreases and improves as the value of d/t increases from zero, but the variation inversely increases as the value of d/t is 0.5 or more. A reason for this can be considered as follows. With the increase of the value of d/t, the electron density largely increases in amount at a location in the processing room  101  below the protrusion  202  due to the deposition of the protrusion  202 . The etching rate locally increases at a portion corresponding to the protrusion  202  for d/t of 0.5 or more, and thus the variation in the etching rate  701  degrades. 
     Next, the relationship between the width of the protrusion  202  or the width w of the recess  203  and the variation in the etching rate, will be described with  FIG. 8 .  FIG. 8  is a graph of the relationship between the ratio (w/φ) between the width w of the recess  203  and the diameter φ of the shower plate  110  (diameter of a portion into which the antenna body  107  and the gas dispersion plate  108  are inserted in the shower plate  110 , in  FIG. 2A ) in the plasma processing apparatus  100  illustrated in  FIG. 1  and the variation in the etching rate  801  in the etching processing performed by the plasma processing apparatus  100 . 
     Here, approximating that the width of the protrusion  202  and the width w of the recess  203  of the shower plate  110  agree with each other or the width w of the recess  203  is slightly larger than the width of the protrusion  202 , the relationship between the diameter φ of the shower plate  110  and the width w of the recess  203  is defined as w/φ. According to the present Example, the diameter of the shower plate  110  was 400 mm. 
     Similarly to  FIG. 7 ,  FIG. 8  illustrates the root-mean-square value (variation) in deviation between the average value of values in the etching rate at positions in the radial direction from the center to the circumferential edge of the wafer  103  acquired in the etching processing to the wafer  103  with the variation in w/φ and the values in the etching rate at the positions. 
     As illustrated in the present figure, it can be seen that as the ratio of the width w of the recess  203  to the diameter φ of the shower plate  110  increases from zero, the variation in the etching rate  801  gradually decreases at up to a value in the ratio, and the variation increases again as the ratio further increases. That is, it can be seen that the variation in the etching rate  801  has a minimum value at a predetermined ratio w/φ. 
     A reason for the variation in the etching rate in the relationship illustrated in  FIG. 8 , can be considered as follows. The region in which the electron density of the plasma  111  increases due to the concentration of the electric field, becomes locally small as the width w of the recess  203  (width of the protrusion  202 ) decreases. The electron density of the plasma  111  increases in a wider region as the width increases. 
     In that point, it can be seen that the ratio between the width w of the recess  203  and the diameter φ of the shower plate  110  has an appropriate positional range in order to effectively reduce the variation in the etching rate  801  in the radial direction of the electron density. With the configuration having no recess  203  formed and no protrusion  202  provided, increase of the electron density in a range wider than the region in which the etching rate drops, degrades the uniformity of the etching rate in comparison to the case of the optimization of the width w of the recess  203 . According to the present Example, as illustrated in  FIG. 8 , setting the ratio between the width w of the recess  203  and the diameter φ of the shower plate  110 , to less than 0.14, decreases the variation in the etching rate  801 . 
     Note that, according to Example described above, the configuration has been described in which the conductive protrusion  202  is electrically connected in contact with the gas dispersion plate  108 , with the conductive protrusion  202  embedded in the recess  203  formed on the shower plate  110 , the conductive protrusion  202  and the gas dispersion plate  108  being separately formed. However, the conductive protrusion  202  and the gas dispersion plate  108  may be integrally formed. 
     As described above, according to Example of the present invention, the variation is reduced in the intensity distribution of the electric field formed in the processing room  101 , in the radial direction from the center to the circumferential edge of the wafer  103 . As a result, the variation is reduced in the electron density in the processing room  101 , in the radial direction of the wafer  103 . Thus, the distribution in the radial direction of the intensity or density of the plasma  111  formed in the processing room  101 , further approximates to uniformity. 
     Furthermore, in the etching processing to the wafer  103  with the plasma  111 , the variation is reduced in a processing characteristic with the plasma, such as the etching rate, between locations on the upper surface of the wafer  103  in the radial direction, so that processing yield improves.