Abstract:
A plasma processing apparatus capable of, over a prolonged period of time, controlling a decrease in the value of a DC current flowing within an accommodating compartment. The plasma processing apparatus comprises an accommodating compartment adapted to accommodate a substrate and perform a plasma treatment thereon, a high-frequency power source adapted to supply high-frequency power to the inside of the accommodating compartment; a DC electrode adapted to apply a DC voltage to the inside of the accommodating compartment, a ground electrode provided within the accommodating compartment and used for the applied DC voltage, and an exhaust unit adapted to evacuate the inside of the accommodating compartment. The plasma processing apparatus further comprises a shielding member disposed in the accommodating compartment so as to extend along the flow of exhaust gas, interpose between the flow of exhaust gas and the ground electrode, and form a cross-sectionally elongated groove-shaped space between the shielding member and the ground electrode.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a plasma processing apparatus and, more particularly, to a plasma processing apparatus having an electrode connected to a DC power source. 
         [0003]    2. Description of the Related Art 
         [0004]    There is known a parallel plate type plasma processing apparatus provided with an accommodating compartment for accommodating a wafer serving as a substrate, a lower electrode disposed within the accommodating compartment and connected to a high-frequency power source, and an upper electrode disposed so as to face the lower electrode. In this plasma processing apparatus, a processing gas is introduced into the accommodating compartment and high-frequency power is supplied into the accommodating compartment. In addition, plasma is produced from the introduced processing gas by the high-frequency power when the wafer is accommodated into the accommodating compartment and mounted on the lower electrode. Thus, a plasma treatment, such as an etching treatment, is performed on the wafer using the plasma, etc. 
         [0005]    In recent years, there has been developed a plasma processing apparatus wherein an upper electrode is connected to a DC power source and a DC voltage is applied to the inside of the accommodating compartment, in order to improve the performance of plasma treatments. In order to apply a DC voltage to the inside of the accommodating compartment, there is the need to provide a ground electrode used for the DC voltage applied to the inside of the accommodating compartment within which a surface of the ground electrode is exposed (hereinafter simply referred to as the “ground electrode”). However, when performing a plasma treatment using a reactive processing gas, a reaction product (deposition) may adhere to a surface of the ground electrode and, therefore, a deposition film may be formed. 
         [0006]    Since the deposition film is insulative, the flow of a DC current from the upper electrode to the ground electrode is blocked, thereby disabling the application of a DC voltage to the inside of the accommodating compartment. As a result, plasma within the accommodating compartment may fall into an unstable state or plasma treatment characteristics may change. 
         [0007]    In consideration of the above, the present inventor has gained the knowledge, through experiments, that a main contributor to the formation of a deposition film is positive ions in plasma and that there is only a small quantity of positive ions in the vicinity of corners formed by component parts of a plasma processing apparatus. Based on the knowledge, the present inventor has proposed preventing a deposition film or the like from being formed on a surface of the ground electrode by disposing a ground electrode in the vicinity of the corners (for example, see Japanese Patent Application No. 2006-081352). 
         [0008]    However, since the ground electrode remains exposed within the accommodating compartment even if disposed in the vicinity of the corners, some positive ions reach the ground electrode and form a deposition film on a surface thereof. The deposition film is formed slowly and, therefore, DC voltage application to the inside of the accommodating compartment is not immediately disabled. However, it has been confirmed by the present inventor that if the total time of a plasma treatment exceeds, for example, 70 hours, the value of a DC current flowing within the accommodating compartment decreases to 1.43 A to 1.33 A. Thus, there is the problem that a decrease in the value of a DC current causes a change in plasma treatment characteristics. 
       SUMMARY OF THE INVENTION 
       [0009]    The present invention provides a plasma processing apparatus which is capable of, over a prolonged period of time, controlling a decrease in the value of a DC current flowing within an accommodating compartment. 
         [0010]    In a first aspect of the present invention, there is provided a plasma processing apparatus comprising: an accommodating compartment adapted to accommodate a substrate and perform a plasma treatment thereon; a high-frequency power source adapted to supply high-frequency power to the inside of the accommodating compartment; a DC electrode adapted to apply a DC voltage to the inside of the accommodating compartment; a ground electrode provided within the accommodating compartment and used for the applied DC voltage; and an exhaust unit adapted to evacuate the inside of the accommodating compartment, wherein the plasma processing apparatus further comprises a shielding member disposed in the accommodating compartment so as to extend along the flow of exhaust gas, interpose between the flow of exhaust gas and the ground electrode, and form a cross-sectionally elongated groove-shaped space between the shielding member and the ground electrode. 
         [0011]    According to the first aspect of the present invention, it is possible to shield the ground electrode from positive ions moving from plasma distributed within the accommodating compartment along the flow of exhaust gas toward component parts including the ground electrode. In addition, since radicals in the plasma easily adhere to members, the radicals adhere to both wall surfaces near the opening of the cross-sectionally long groove-shaped space and hardly enter the groove-shaped space. Consequently, the positive ions and radicals do not reach the ground electrode in the groove-shaped space. As a result, any deposition films attributable to the positive ions and radicals are not formed on the ground electrode over a prolonged period of time. On the other hand, electrons in the plasma move freely and, therefore, enter the groove-shaped space and reach the ground electrode. Hence, it is possible, over a prolonged period of time, to keep electrons reachable to the ground electrode. Thus, it is possible, over a prolonged period of time, to control a decrease in the value of a DC current flowing within the accommodating compartment. 
         [0012]    A gap between the ground electrode and the shielding member forming the groove-shaped space can be greater than 0.5 mm. 
         [0013]    According to the first aspect of the present invention, the opening of the groove-shaped space can be faced with the plasma. This is because the thickness of a sheath present between the plasma and component parts is, in general, approximately 0.5 mm. Consequently, it is possible to let electrons move from the plasma through the opening to the ground electrode. Thus, it is possible to reliably flow a DC current into the accommodating compartment. 
         [0014]    The gap can be not less than 2.5 mm but not greater than 5.0 mm. 
         [0015]    According to the first aspect of the present invention, electrons are not blocked from entering the groove-shaped space, whereas radicals are prevented from entering thereinto. Consequently, it is possible, over a prolonged period of time, to reliably control a decrease in the value of a DC current flowing within the accommodating compartment. 
         [0016]    The gap can be not less than 3.5 mm. 
         [0017]    According to the first aspect of the present invention, a gap between the ground electrode and the shielding member forming the groove-shaped space is 3.5 mm or greater. The opening of the groove-shaped space therefore widens, thereby allowing electrons to smoothly enter the groove-shaped space and enabling the prevention of the occurrence of plasma fluctuations. 
         [0018]    An aspect ratio in a cross section of the groove-shaped space can be not less than 3.0. 
         [0019]    According to the first aspect of the present invention, radicals adhere to both wall surfaces near the opening of the cross-sectionally long groove-shaped space before entering deep thereinto. As a result, the radicals do not enter deep into the groove-shaped space. Thus, it is possible, over a prolonged period of time, to prevent the entire surface of the ground electrode from being covered with a deposition film. 
         [0020]    The edge of the shielding member on the opening side of the groove-shaped space can protrude along the flow of exhaust gas from the edge of the ground electrode on the opening side. 
         [0021]    According to the first aspect of the present invention, radicals trying to enter the groove-shaped space through the opening thereof can be made to actively adhere to the shielding member. As a result, it is possible, over a prolonged period of time, to prevent the entire surface of the ground electrode from being covered with a deposition film. 
         [0022]    The amount of protrusion of the edge of the shielding member on the opening side from the edge of the ground electrode on the opening side can be not greater than 3 mm. 
         [0023]    According to the first aspect of the present invention, the protruding part of the shielding member can inhibit the blockage of electrons from entering the groove-shaped space. Thus, it is possible to prevent the occurrence of plasma fluctuations. 
         [0024]    In a second aspect of the present invention, there is provided a plasma processing apparatus comprising: an accommodating compartment adapted to accommodate a substrate and perform a plasma treatment thereon; a high-frequency power source adapted to supply high-frequency power to the inside of the accommodating compartment; a DC electrode adapted to apply a DC voltage to the inside of the accommodating compartment; a ground electrode exposed to the surfaces of component parts within the accommodating compartment and used for the applied DC voltage provided; and an exhaust unit adapted to evacuate the inside of the accommodating compartment, wherein the plasma processing apparatus further comprises a shielding member disposed in the accommodating compartment so as to extend along the surfaces of the component parts and form a cross-sectionally elongated groove-shaped space between the shielding member and the ground electrode. 
         [0025]    According to the second aspect of the present invention, plasma is distributed along surfaces of component parts within the accommodating compartment and positive ions move from the plasma to the component parts including the ground electrode. Since the shielding member is disposed so as to locate along the surfaces of the component parts, the shielding member shields the ground electrode from moving positive ions. In addition, since radicals in the plasma easily adhere to members, the radicals adhere to both wall surfaces near the opening of the cross-sectionally long groove-shaped space and hardly enter the groove-shaped space. Consequently, the positive ions and radicals do not reach the ground electrode in the groove-shaped space. As a result, any deposition films attributable to the positive ions and radicals are not formed on the ground electrode over a prolonged period of time. On the other hand, electrons in the plasma move freely and, therefore, enter the groove-shaped space and reach the ground electrode. Hence, it is possible, over a prolonged period of time, to keep electrons reachable to the ground electrode. Accordingly, it is possible, over a prolonged period of time, to control a decrease in the value of a DC current flowing within the accommodating compartment. 
         [0026]    The above and other objects, features, and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]      FIG. 1  is a cross-sectional view illustrating a schematic configuration of a plasma processing apparatus in accordance with an embodiment of the present invention. 
           [0028]      FIG. 2  is an enlarged cross-sectional view illustrating schematic configurations of a grounding ring and a shielding member for shielding the grounding ring. 
           [0029]      FIG. 3  is a schematic view used to explain the relationship between the thickness of a sheath and a gap between a ground electrode surface and a shielding member. 
           [0030]      FIG. 4  is a graph showing how the presence/absence of a shielding member affects the rate of decrease in the value of a DC current flowing within a processing space. 
           [0031]      FIG. 5  is a graph showing how a gap between a ground electrode surface and a shielding member affects the rate of decrease in the value of a DC current flowing within a processing space. 
           [0032]      FIG. 6  is a table summarizing the result of observing plasma fluctuations in a plasma processing apparatus provided with a new grounding ring, while changing a gap between a ground electrode surface and a shielding member and the protruding amount of the edge of the shielding member to various values. 
           [0033]      FIG. 7  is a table summarizing the result of observing plasma fluctuations in a plasma processing apparatus wherein an etching treatment has been performed for 50 hours, while changing a gap between a ground electrode surface and a shielding member and the protruding amount of the edge of the shielding member to various values. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0034]    Preferred embodiments of the present invention will be now described in detail with reference to the drawings. 
         [0035]      FIG. 1  is a cross-sectional view illustrating a schematic configuration of a plasma processing apparatus in accordance with an embodiment of the present invention. This plasma processing apparatus is configured to perform an etching treatment on a semiconductor wafer W serving as a substrate. 
         [0036]    In  FIG. 1 , a plasma processing apparatus  10  includes an approximately cylindrical accommodating compartment  11  for accommodating a semiconductor wafer W (hereinafter simply referred to as the “wafer W”) and the accommodating compartment  11  includes a processing space PS (within the accommodating compartment) formed upwardly therein. In the processing space PS, there is produced plasma to be explained later. In addition, a cylindrical susceptor  12  serving as a mounting stage to be mounted with a wafer W is disposed within the accommodating compartment  11 . The inner side wall surface of the accommodating compartment  11  is covered with aside wall member  13  and the inner upper wall surface thereof is covered with an upper wall member  14 . The side wall member  13  and the upper wall member  14  are made of aluminum and the surfaces thereof facing the processing space PS are coated with yttria or alumite having a predetermined thickness. Since the accommodating compartment  11  is electrically grounded, the side wall member  13  and the upper wall member  14  are at a ground potential. In addition, the susceptor  12  includes a conductor part  15  made of a conductive material, such as aluminum, a side covering member  16  (component part) made of an insulating material and used to cover the side surface of the conductor part  15 , and an enclosure member  17  made of quartz (Qz) and mounted on the side covering member  16 . 
         [0037]    In the plasma processing apparatus  10 , an exhaust flow passage  18  (within the accommodating compartment) functioning as a flow path for exhausting gases in the processing space PS out of the accommodating compartment  11  is formed by the inner side wall of the accommodating compartment  11  and the side wall of the susceptor  12 . In this exhaust flow passage  18 , there is disposed an exhaust plate  19  which is a plate-like member having a multitude of vent holes. The exhaust plate  19  divides off an exhaust space ES which is the lower space of the exhaust flow passage  18  and the accommodating compartment  11 . Note here that the exhaust flow passage  18  causes the exhaust space ES and the processing space PS to be communicated with each other. In addition, a roughing exhaust pipe  20  and a main exhaust pipe  21  are open into the exhaust space ES. A dry pump (DP) (not shown) is connected to the roughing exhaust pipe  20  and a turbo molecular pump (TMP) (not shown) is connected to the main exhaust pipe  21 . 
         [0038]    The roughing exhaust pipe  20 , the main exhaust pipe  21 , the DP, the TMP, and the like configure an exhaust unit. The roughing exhaust pipe  20  and the main exhaust pipe  21  exhaust gases in the processing space PS out of the accommodating compartment  11  through the exhaust flow passage  18  and the exhaust space ES. Specifically, the roughing exhaust pipe  20  depressurizes the processing space PS from an atmospheric pressure to a low-vacuum state, and the main exhaust pipe  21 , in conjunction with the roughing exhaust pipe  20 , depressurizes the processing space PS from an atmospheric pressure to a high-vacuum state (for example, 133 Pa (1 Torr) or lower) which is lower than that of the low-vacuum state. 
         [0039]    A high-frequency power source  22  is connected to the conductor part  15  of the susceptor  12  through a matching box  23 . The high-frequency power source  22  supplies high-frequency power having a relatively high frequency, such as 40 MHz, to the conductor part  15 . Consequently, the susceptor  12  functions as a high-frequency electrode and supplies 40 MHz high-frequency power to the processing space PS. Note that the matching box  23  reduces the reflection of the high-frequency power from the conductor part  15  and maximizes the efficiency of high-frequency power supply to the conductor part  15 . 
         [0040]    Another high-frequency power source  24  is connected to the conductor part  15  through a matching box  25 . The additional high-frequency power source  24  supplies high-frequency power having a frequency, such as 2 MHz, lower than that of the high-frequency power supplied by the high-frequency power source  22 , to the conductor part  15 . 
         [0041]    On the susceptor  12 , there is disposed an electrostatic chuck  27  containing an electrostatic electrode plate  26 . The electrostatic chuck  27  is formed of a lower disc-shaped member having a specific diameter on which there is placed an upper disc-shaped member having a diameter less than the diameter of the lower disc-shaped member. A lower DC power source  28  is electrically connected to the electrostatic electrode plate  26 . When the susceptor  12  is mounted with a wafer W, the wafer W is placed on the electrostatic chuck  27 . At this time, a positive potential is generated on the back side of the wafer W when a negative DC voltage is applied to the electrostatic electrode plate  26 . Consequently, there arises a potential difference between the electrostatic electrode plate  26  and the back side of the wafer W. Thus, the wafer W is sucked and held onto the upper surface of the electrostatic chuck  27  by a coulomb force or a Johnson-Rahbeck force caused by the potential difference. 
         [0042]    In addition, an annular focus ring  29  is disposed on the susceptor  12 , so as to surround the wafer W sucked and held onto the upper surface of the susceptor  12 . This focus ring  29  is made of silicon (Si) or silica (SiO 2 ) and exposed to the processing space PS, in order to converge plasma in the processing space PS onto the surface of the wafer W and improve the efficiency of etching treatments. In addition, an annular cover ring  30  made of quartz and used to protect the side surface of the focus ring  29  is disposed around the focus ring  29 . 
         [0043]    Within the susceptor  12 , there is provided, for example, an annular refrigerant chamber  31  extending in the circumference direction of the susceptor  12 . A refrigerant with a predetermined temperature, for example, cooling water or a Galden (registered trademark) fluid is cyclically supplied from a chiller unit (not shown) to this refrigerant chamber  31  through a refrigerant pipe  32 . The treatment temperature of the wafer W sucked and held onto the upper surface of the susceptor  12  is controlled by the refrigerant. 
         [0044]    Furthermore, a plurality of heat-transmitting gas supply holes  33  are created in a part of the upper surface of the susceptor  12  onto which the wafer W is sucked and held (hereinafter referred to as the “sucking surface”). This plurality of heat-transmitting gas supply holes  33  are connected to a heat-transmitting gas supply unit (not shown) through a heat-transmitting gas supply line  34  disposed within the susceptor  12 . The heat-transmitting gas supply unit supplies a helium (He) gas serving as a heat-transmitting gas to a gap between the sucking surface and the back side of the wafer W through the heat-transmitting gas supply holes  33 . 
         [0045]    In addition, a plurality of pusher pins  35  serving as lift pins capable of freely protruding from the upper surface of the susceptor  12  are disposed on the sucking surface of the susceptor  12 . These pusher pins  35  freely protrude from the sucking surface. When sucking and holding the wafer W onto the sucking surface in order to perform an etching treatment on the wafer W, the pusher pins  35  are retracted into the susceptor  12 . When the wafer W having been subjected to the etching treatment is transferred out of the accommodating compartment  11 , the pusher pins  35  protrude from the sucking surface to uplift the wafer W. 
         [0046]    A shower head  36  is disposed on the ceiling part of the accommodating compartment  11 , so as to face the susceptor  12 . The shower head  36  is provided with a disc-shaped cooling plate  38  made of an insulating material within which a buffer compartment  37  is formed, an upper electrode plate  39  suspended by the cooling plate  38 , and a lid body  40  for covering the cooling plate  38 . The upper electrode plate  39 , the bottom surface of which is exposed to the processing space PS, is a disc-shaped member made of a conductive material, such as silicon. The peripheral part of the upper electrode plate  39  is covered with an annular shield ring  41  made of an insulating material. That is, the upper electrode plate  39  is electrically insulated by the cooling plate  38  and the shield ring  41  from the walls of the accommodating compartment  11  placed at a ground potential. 
         [0047]    The upper electrode plate  39  is electrically connected to a DC power source  42  and a negative DC voltage is applied to upper electrode plate  39 . Consequently, the upper electrode plate  39  applies a DC voltage to the processing space PS. Since a DC voltage is applied to the upper electrode plate  39 , there is no need to interpose a matching box between the upper electrode plate  39  and the upper DC power source  42 . Thus, it is possible to make the structure of the plasma processing apparatus  10  simpler, compared with a case wherein a high-frequency power source is connected to an upper electrode plate through a matching box as in a conventional plasma processing apparatus. 
         [0048]    A processing gas introduction pipe  43  leading from a processing gas supply unit (not shown) is connected to the buffer compartment  37  of the cooling plate  38 . In addition, the shower head  36  includes a plurality of through gas holes  44  whereby the buffer compartment  37  is communicated with the processing space PS. The shower head  36  supplies a processing gas, which is supplied from the processing gas introduction pipe  43  to the buffer compartment  37 , to the processing space PS by way of the through gas holes  44 . 
         [0049]    The plasma processing apparatus  10  further includes a cross-sectionally L-shaped annular grounding ring  45  (ground electrode) disposed in the exhaust flow passage  18 . The grounding ring  45  is made of a conductive material, such as silicon, and functions as a ground electrode used for a DC voltage applied by the upper electrode plate  39 . In addition, the grounding ring  45  is disposed so as to cover the side surface of a susceptor base  15   a  below the side covering member  16  of the susceptor  12 . That is, the grounding ring  45  appears as if it is exposed to a surface of the side covering member  16  when the susceptor  12  is viewed from a side thereof assuming that a shielding member  46  to be described later is not present. Electrons released from the upper electrode plate  39  reach this grounding ring  45 , thereby causing a DC current to flow through the processing space PS. 
         [0050]    In this plasma processing apparatus  10 , high-density plasma is produced in the processing space PS from a processing gas supplied from the shower head  36  by supplying high-frequency power to the processing space PS. In addition, the produced plasma is maintained in a desired condition by the DC current of the plasma processing space PS to perform an etching treatment on a wafer W using the plasma. 
         [0051]    Incidentally, radicals which are activated neutral particles, positive ions and electrons are mixed in the produced plasma. If positive ions and radicals, particularly CF-based positive ions and radicals, reach and adhere to the grounding ring  45 , a deposition film may be formed on a surface of the grounding ring  45 . 
         [0052]    In general, in order to prevent the surfaces of members from being covered with a deposition film, there is used either (1) a method of removing the deposition film by sputtering using ions or (2) a method of preventing positive ions and radicals from reaching the surfaces of members. Note here that method  1  requires providing a sputtering step and, therefore, a step of wafer etching treatment becomes complicated. In addition, there is the possibility that a deposition sputtered and separated from the surfaces of members turn particles. Hence, in the present embodiment, method  2  is used to prevent a deposition film from being formed on the surface of the grounding ring  45 . 
         [0053]    Note here that positive ions, radicals and electrons in plasma have the movement characteristics described below. 
         [0054]    Positive ions: Move from the plasma toward members facing the plasma. 
         [0055]    Radicals: Easy to be trapped onto the surfaces of members. This tendency is remarkable in highly reactive radicals. 
         [0056]    Electrons: Have no directionality in movement and, therefore, individual electrons move freely. 
         [0057]    In the present embodiment, the grounding ring  45  is shielded from positive ions moving from plasma and a member for trapping radicals is provided in the vicinity of the grounding ring  45  in consideration of the movement characteristics of the above-described positive ions, radicals and electrons. Specifically, there is provided a shielding member  46  used to shield the grounding ring  45  shown in  FIG. 1  and described in detail below in the exhaust flow passage  18  of the plasma processing apparatus  10 . 
         [0058]      FIG. 2  is an enlarged cross-sectional view illustrating schematic configurations of the grounding ring and the shielding member for shielding the grounding ring. 
         [0059]    In  FIG. 2 , the annular shielding member  46  is mounted on the lower extension part  45   a  of the cross-sectionally L-shaped grounding ring  45 . The shielding member  46  is made of an insulating material, such as quartz, and is disposed concentrically with the grounding ring  45 . In addition, the shielding member  46  is disposed in a cross section along the radial direction of the grounding ring  45  (shielding member  46 ) (i.e., the cross section shown in  FIG. 2 ), so as to locate along the surface of the side covering member  16 , thereby forming a cross-sectionally long groove-shaped space  47  between the shielding member  46  and the grounding ring  45 . Note here that since the flow of exhaust gas (shown by an outline arrow in the figure) in the exhaust flow passage  18  is directed along the surface of the side covering member  16 , the shielding member  46  is also directed along the flow of exhaust gas in the above-described cross section. Since the groove-shaped space  47  is sandwiched by the grounding ring  45  exposed to the surface of the side covering member  16  and the shielding member  46  directed along the flow of exhaust gas, the groove-shaped space  47  is also directed along the flow of exhaust gas. Thus, the shielding member  46  interposes between the flow of exhaust gas and the grounding ring  45 . Note that the groove-shaped space  47  is open toward the upstream of the flow of exhaust gas. 
         [0060]    In addition, the edge  46   a  of the shielding member  46  on the opening side of the groove-shaped space  47  (hereinafter simply referred to as the “opening side”) protrudes along the flow of exhaust gas more than the edge  45   b  of the grounding ring  45  on the opening side thereof. Specifically, the edge  46   a  protrudes toward the upstream of the flow of exhaust gas. 
         [0061]    In the exhaust flow passage  18 , plasma  48  is distributed along the flow of exhaust gas, as well as along the surfaces of component parts such as the side covering member  16 . Positive ions shown by “O” in the figure move from the plasma  48  toward the side covering member  16  and the grounding ring  45 . Since the shielding member  46  interposes between the flow of exhaust gas (i.e., the plasma  48 ) and the grounding ring  45 , the shielding member  46  shields the grounding ring  45  from the moving positive ions. 
         [0062]    In addition, radicals shown by “Δ” in the figure try to move from the plasma  48  to enter the groove-shaped space  47 . Since the radicals generally move along the flow of exhaust gas, they can hardly enter the groove-shaped space  47  which is only open toward the upstream of the flow of exhaust gas. Furthermore, since the radicals are easy to be trapped onto the surfaces of members, they adhere to both wall surfaces near the opening of the groove-shaped space  47 . As a result, the radicals hardly enter the groove-shaped space  47 . In the plasma processing apparatus  10  in particular, the edge  46   a  of the shielding member  46  protrudes toward the upstream of the flow of exhaust gas more than the edge  45   b  of the grounding ring  45  on the opening side thereof, the radicals actively adhere to the shielding member  46 . 
         [0063]    Consequently, positive ions and radicals do not reach the ground electrode surface  45   c  of the grounding ring  45  in the groove-shaped space  47 . As a result, any deposition films attributable to the positive ions and radicals are not formed on the ground electrode surface  45   c  over a prolonged period of time. 
         [0064]    On the other hand, since electrons shown by “X” in the figure move freely, they move from the plasma  48  to enter the groove-shaped space  47  and reach the ground electrode surface  45   c . Accordingly, it is possible to flow a DC current through the processing space PS and the exhaust flow passage  18 . 
         [0065]    Incidentally, in the plasma processing apparatus  10 , if a gap “t” between the ground electrode surface  45   c  of the grounding ring  45  and the shielding member  46  forming the groove-shaped space  47  is set to a value less than the thickness “ts” of a sheath  49  present between the side covering member  16  and the plasma  48 , as shown in  FIG. 3 , then the opening of the groove-shaped space  47  cannot be faced with the plasma  48 . As a result, electrons in the plasma  48  (shown by “X” in the figure) can hardly enter the groove-shaped space  47  and do not reach the ground electrode surface  45   c . Thus, a DC current does not flow through the processing space PS and the exhaust flow passage  18 . 
         [0066]    Hence, in the present embodiment, the gap “t” between the ground electrode surface  45   c  and the shielding member  46  is set to a value greater than the thickness “ts” of the sheath  49 . Consequently, the opening of the groove-shaped space  47  can be faced with the plasma  48 . Note that since, in general, the thickness of a sheath is approximately 0.5 mm, the gap “t” is set to a value greater than 0.5 mm. 
         [0067]    In addition, from the viewpoint of preventing the breakage of the shielding member  46  and facilitating the handling thereof, the thickness of the shielding member  46  in the radial direction thereof is set to, for example, 5 mm or greater at which the rigidity of the shielding member  46  can be ensured. 
         [0068]    According to the plasma processing apparatus  10  in accordance with the present invention, the shielding member  46  is disposed in the exhaust flow passage  18 , so as to locate along the surfaces of the side covering member  16  and the like and along the flow of exhaust gas, interpose between the flow of exhaust gas and the grounding ring  45 , and form the cross-sectionally long groove-shaped space  47  between the shielding member  46  and the grounding ring  45 . Consequently, it is possible to keep electrons reachable to the ground electrode surface  45   c  over a prolonged period of time. Thus, it is possible, over a prolonged period of time, to control a decrease in the value of a DC current flowing through the processing space PS and the exhaust flow passage  18 . 
         [0069]    In addition, in the plasma processing apparatus  10 , the edge  46   a  of the shielding member  46  protrudes toward the upstream of the flow of exhaust gas more than the edge  45   b  of the grounding ring  45  on the opening side thereof. Consequently, it is possible to cause radicals trying to enter from the opening into the groove-shaped space  47  to actively adhere to the shielding member  46  near the opening of the groove-shaped space  47 . As a result, it is possible, over a prolonged period of time, to prevent the ground electrode surface  45   c  from being covered with a deposition film. 
         [0070]    Furthermore, in the plasma processing apparatus  10 , since the gap “t” between the ground electrode surface  45   c  and the shielding member  46  is set to a value greater than 0.5 mm, the opening of the groove-shaped space  47  can be faced with the plasma  48 . Thus, it is possible to let electrons move from the plasma  48  to the ground electrode surface  45   c  through the opening, thereby enabling a DC current to reliably flow through the processing space PS and the exhaust flow passage  18 . 
         [0071]    Although the shielding member  46  is mounted on the lower extension part  45   a  of the grounding ring  45 , an engagement part for engaging with the periphery of the grounding ring  45  may be provided in the shielding member  46  to engage the grounding ring  45  and the shielding member  46  with each other. 
         [0072]    Although the grounding ring  45  and the shielding member  46  are provided near the exhaust plate  19  of the exhaust flow passage  18 , the grounding ring  45  and the shielding member  46  may be provided anywhere in the processing space PS or in the exhaust flow passage  18 , as long as they are close to the plasma. However, the shielding member for forming the cross-sectionally long groove-shaped space in conjunction with the grounding ring must be located along the flow of exhaust gas or along the surfaces of component parts. 
         [0073]    In addition, although the grounding ring  45  is formed of silicon, the grounding ring may be formed of silicon carbide. Likewise, the shielding member  46  may be formed not only of quartz but also of a metal material onto the surface of which an insulating film has been flame-sprayed. 
         [0074]    Furthermore, the ground electrode is not limited to an annular member such as the grounding ring  45 . Alternatively, the ground electrode may be formed of a plurality of conductive members disposed around the susceptor  12 . 
         [0075]    Although in the above-described plasma processing apparatus  10 , two types of high-frequency power are supplied to the conductor part  15  of the susceptor  12 , one type of high-frequency power may be supplied to the conductor part  15  of the susceptor  12  and the upper electrode plate  39  of the shower head  36 , respectively. Also in this case, the same advantageous effect as described above can be obtained. 
         [0076]    Next, examples of the present invention will be described. 
         [0077]    First, the present inventor confirmed how the presence/absence of the shielding member  46  in the plasma processing apparatus  10  affected the rate of decrease (degradation rate) in the value of a DC current flowing within the processing space PS. 
       Example 1 
       [0078]    In the plasma processing apparatus  10 , a gap “t” between the ground electrode surface  45   c  of a grounding ring  45  and a shielding member  46  was set to 2.5 mm, and the amount of protrusion “T” of the edge  46   a  of the shielding member  46  on the opening side thereof from the edge  45   b  of the grounding ring  45  on the opening side thereof (see FIG.  2 —hereinafter simply referred to as the “protruding amount “T” of the shielding member  46 ”) was set to 0 mm. 
         [0079]    After that, an etching treatment on a wafer W was repeated in the plasma processing apparatus  10 . In each case of etching treatment, a DC current flowing through a processing space PS was measured and the measured values of the DC current were shown by “X” in the graph of  FIG. 4 . Then, a calculation was made of an approximate expression of the rate of decrease in the value of the DC current (hereinafter simply referred to as the “decrease rate”) for Example 1 in the graph of  FIG. 4 , thus obtaining Equation (1) shown below: 
         [0000]        DC  current value=−1.75×10 −5 ×number of treated wafers+1.33  (1) 
         [0000]    where, the term “−1.75×10 −5 ”, corresponds to the decrease rate. 
       Example 2 
       [0080]    In a plasma processing apparatus  10 , a gap “t” was set to 3.5 mm and the protruding amount “T” of a shielding member  46  was set to 3.0 mm. 
         [0081]    After that, an etching treatment on a wafer W was repeated in the plasma processing apparatus  10 , as in Example 1. In each case of etching treatment, a DC current flowing through a processing space PS was measured and the measured values of the DC current were shown by “Δ” in the graph of  FIG. 4 . Then, a calculation was made of an approximate decrease rate expression for Example 2, thus obtaining Equation (2) shown below 
         [0000]        DC  current value=−6.04×10 −6 ×number of treated wafers+1.39  (2) 
         [0082]    where, the term “−6.04×10 −6 ” corresponds to the decrease rate (degradation rate). 
       Comparative Example 1 
       [0083]    In the plasma processing apparatus  10 , the shielding member  46  was removed. Then, an etching treatment on a wafer W was repeated in the plasma processing apparatus  10 , as in Example 1. In each case of etching treatment, a DC current flowing through a processing space PS was measured and the measured values of the DC current were shown by “O” in the graph of  FIG. 4 . Then, a calculation was made of an approximate decrease rate expression for Comparative Example 1 in the graph of  FIG. 4 , thus obtaining Equation (3) shown below: 
         [0000]        DC  current value=−1.21×10 −4 ×number of treated wafers+1.44  (3) 
         [0000]    where, the term “−1.21×10 −4 ” corresponds to the decrease rate (degradation rate). 
         [0084]    A comparison made among decreases in a DC current in Examples 1 and 2 and Comparative Example 1 in the graph of  FIG. 4  proved that the decrease rate of Example 1 was improved to approximately 1/7 the decrease rate of Comparative Example 1 and the decrease rate of Example 2 was improved to approximately 1/20 the decrease rate of Comparative Example 1. 
         [0085]    Next, the present inventor confirmed how the gap “t” affected the decrease rate of a DC current. 
       Example 3 
       [0086]    In a plasma processing apparatus  10 , a gap “t” was set to 4.0 mm and the protruding amount “T” of a shielding member  46  was set to 3.0 mm. 
         [0087]    After that, an etching treatment on a wafer W was repeated in the plasma processing apparatus  10 , as in Example 1. In each case of etching treatment, a DC current flowing through a processing space PS was measured and the measured values of the DC current were shown by “□” in the graph of  FIG. 5 . 
       Example 4 
       [0088]    In a plasma processing apparatus  10 , a gap “t” was set to 5.0 mm and the protruding amount “T” of a shielding member  46  was set to 3.0 mm. 
         [0089]    After that, an etching treatment on a wafer W was repeated in the plasma processing apparatus  10 , as in Example 1. In each case of etching treatment, a DC current flowing through a processing space PS was measured and the measured values of the DC current were shown by “X” in the graph of  FIG. 5 . 
         [0090]    The measured values of a DC current in Example 2 (where the gap “t” was 3.5 mm) were also shown by “A” in the graph of  FIG. 5 . 
         [0091]    A comparison made among DC current values in Examples 2, 3 and 4 in the graph of  FIG. 5  proved that the DC current values hardly changed at all in either example and that the decrease rate of a DC current was almost 0 in either example. 
         [0092]    As described above, it proved from Examples 1 to 4 that a decrease in the value of a DC current flowing through the processing space PS could be controlled over a prolonged period of time as long as the gap “t” was not less than 2.5 mm but not greater than 5.0 mm. The reason for this was assumed to be that electrons were not blocked from entering the groove-shaped space  47 , whereas radicals were prevented from entering thereinto, by setting the gap “t” to a value from 2.5 mm to 5.0 mm. 
         [0093]    It becomes easier for radicals to enter the groove-shaped space  47  as the gap “t” becomes greater. Thus, there arises the possibility that a deposition film is formed on the ground electrode surface  45   c  and the value of a DC current flowing through the processing space PS is decreased. Since the DC current hardly changed at all, as shown in Example 4, even if the gap “t” was 5.0 mm, it has proven that radicals adhere to both wall surfaces near the opening of the groove-shaped space  47  before entering deep into the groove-shaped space  47  and reach the ground electrode surface  45   c , if the gap “t” is at least 5.0 mm. Note here that since a length L (see  FIG. 2 ) along the flow of exhaust gas in the shielding member  46  is 15 mm, an aspect ratio in a cross section of the groove-shaped space  47  is 3.0. Consequently, radicals do not enter deep into the groove-shaped space  47  if the aspect ratio in the cross section of the groove-shaped space  47  is 3.0 or greater. Thus, it has proven that it is possible, over a prolonged period of time, to prevent the entire surface of the ground electrode surface  45   c  from being covered with a deposition film. 
         [0094]    Incidentally, if the gap “t” becomes less or an aspect ratio in a cross section of the groove-shaped space  47  becomes greater (i.e., the protruding amount “T” of the shielding member  46  becomes greater), it becomes difficult for not only radicals but also electrons to enter the groove-shaped space  47 . If electrons can hardly enter the groove-shaped space  47 , then a DC current can hardly flow through the processing space PS. Hence, the present inventor examined how the gap “t” and the protruding amount “T” affected the easiness to flow of a DC current. 
         [0095]    Specifically, since plasma in the processing space PS fluctuated if a DC current could hardly flow in the processing space PS, the present inventor observed plasma fluctuations while changing the values of the gap “t” and the protruding amount “T” to various values. 
       Examples 5 to 13 
       [0096]    First, plasma fluctuations were observed in a plasma processing apparatus  10  provided with a new grounding ring  45 , while changing the values of the gap “t” and the protruding amount “T” to various values during etching treatment. The results of observation were summarized in a table shown in  FIG. 6 . 
         [0097]    From the table of  FIG. 6 , it proved that the greater the gap “t” became, the less plasma fluctuations were likely to occur and that plasma fluctuations hardly occurred if the gap “t” was 3.5 mm. The reason for this was assumed to be that the opening of a space formed by the grounding ring  45  and the shielding member  46  widened and electrons could smoothly enter the space. 
         [0098]    It also proved that if the space formed by the grounding ring  45  and the shielding member  46  was cross-sectionally L-shaped, plasma fluctuations were more likely to occur, whereas plasma fluctuations did not occur if the space was at least groove-shaped. 
         [0099]    As described heretofore, it has proven that if the grounding ring  45  and the shielding member  46  form a groove-shaped space and the gap “t” between the grounding ring  45  and the shielding member  46  is not less than 3.5 mm, it is possible to prevent the occurrence of plasma fluctuations. 
       Examples 14 to 20 
       [0100]    Next, after performing an etching treatment on a wafer W for 50 hours in a plasma processing apparatus  10 , the present inventor observed plasma fluctuations, while changing the values of a gap “t” and a protruding amount “T” to various values during the etching treatment. Then, the present inventor summarized the observation results in a table shown in  FIG. 7 . 
         [0101]    From the table of  FIG. 7 , it proved that the greater the protruding amount “T” became, the more plasma fluctuations were likely to occur. The reason for this was assumed to be that if the protruding amount “T” was too large, the edge  46   a  of a shielding member  46  blocked electrons from entering a groove-shaped space  47 . In particular, plasma fluctuations occurred if the protruding amount “T” was 6.5 mm in a case where the gap “t” was 4.0 mm, whereas plasma fluctuations did not occur if the protruding amount “T” was 3 mm. Hence, it proved that the protruding amount “T” was preferably 3 mm or smaller. 
         [0102]    As described heretofore, it has proven that it is possible to prevent the occurrence of plasma fluctuations if the protruding amount “T” is not greater than 3 mm. 
         [0103]    Note that it has also been confirmed in the table of  FIG. 7 , as in the table of  FIG. 6 , that the greater the gap “t” becomes, the less plasma fluctuations are likely to occur. 
         [0104]    While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all modifications, equivalent structures and functions. 
         [0105]    This application claims priority from Japanese Patent Application No. 2007-089804 filed Mar. 29, 2007, which is hereby incorporated by reference herein in its entirety.