Patent Publication Number: US-2022238313-A1

Title: Apparatus for plasma processing and method of etching

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of U.S. patent application Ser. No. 16/725,915 filed Dec. 23, 2019, which is based on and claims the benefit of priority from Japanese Patent Application No. 2019-001662 filed on Jan. 9, 2019 and Japanese Patent Application No. 2019-217048 filed on Nov. 29, 2019, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD 
     Exemplary embodiments of the present disclosure relate to an apparatus for plasma processing and a method of etching. 
     BACKGROUND 
     In plasma etching of a substrate, a plasma processing apparatus is used. The plasma processing apparatus is provided with a chamber, an electrostatic chuck, and a lower electrode. The electrostatic chuck and the lower electrode are provided in the chamber. The electrostatic chuck is provided on the lower electrode. The electrostatic chuck supports a focus ring placed thereon. The electrostatic chuck supports a substrate disposed in a region surrounded by the focus ring. When etching is performed in the plasma processing apparatus, a gas is supplied into the chamber. Further, radio frequency power is supplied to the lower electrode. A plasma is formed from the gas in the chamber. The substrate is etched by chemical species such as ions and radicals from the plasma. 
     If the plasma etching is performed, the focus ring wears down, so that the thickness of the focus ring is reduced. If the thickness of the focus ring is reduced, a position of an upper end of a plasma sheath (hereinafter referred to as a “sheath”) above the focus ring is lowered. The position in a vertical direction of the upper end of the sheath above the focus ring should be equal to the position in the vertical direction of the upper end of the sheath above the substrate. Therefore, Japanese Patent Application Laid-Open Publication No. 2007-258417 and Japanese Patent Application Laid-Open Publication No.2010-283028 disclose plasma processing apparatuses making it possible to adjust the position in the vertical direction of the upper end of the sheath above the focus ring. The plasma processing apparatuses disclosed in these literatures are configured to apply a direct-current voltage to the focus ring. 
     SUMMARY 
     In an exemplary embodiment, an apparatus for plasma processing is provided. The apparatus includes a chamber, a substrate support, a sheath adjuster, a radio frequency power source, a bias power source, and a controller. The substrate support has a lower electrode and an electrostatic chuck. The electrostatic chuck is provided on the lower electrode. The substrate support is configured to support a substrate which is placed thereon in the chamber. The sheath adjuster is configured to adjust a position in a vertical direction of an upper end of a sheath above an edge ring. The edge ring is disposed to surround an edge of the substrate. The radio frequency power source is configured to generate radio frequency power which is supplied to generate a plasma from a gas in the chamber. The radio frequency power has a first frequency. The bias power source is configured to generate bias power. The bias power source is electrically connected to the lower electrode. The bias power is set to vary a potential of the substrate placed on the electrostatic chuck within a cycle which is defined at a second frequency. The second frequency is lower than the first frequency. The controller is configured to control the sheath adjuster and the radio frequency power source. The controller is configured to control the radio frequency power source to supply the radio frequency power in at least a part of a first period in the cycle. In the first period, the potential of the substrate placed on the electrostatic chuck is higher than an average value of the potential of the substrate in the cycle. The controller is configured to control the radio frequency power source to reduce, in a second period in the cycle, a power level of the radio frequency power to be lower than a power level of the radio frequency power in the first period. In the second period, the potential of the substrate placed on the electrostatic chuck is lower than the average value of the potential of the substrate in the cycle. The controller is configured to control the sheath adjuster to adjust the position in the vertical direction of the upper end of the sheath in the first period and the second period. 
     The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, exemplary embodiments, and features described above, further aspects, exemplary embodiments, and features will become apparent by reference to the drawings and the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically illustrates a plasma processing apparatus according to an exemplary embodiment. 
         FIG. 2  is a timing chart showing radio frequency power and bias power which are used in a plasma processing apparatus according to an exemplary embodiment. 
         FIG. 3A  illustrates an example of a position in a vertical direction of an upper end of a sheath in a state where an edge ring has worn down, and  FIG. 3B  illustrates an example of a corrected position in the vertical direction of the upper end of the sheath. 
         FIG. 4  is a timing chart showing radio frequency power and bias power which are used in a plasma processing apparatus according to another exemplary embodiment. 
         FIG. 5  schematically illustrates a plasma processing apparatus according to still another exemplary embodiment. 
         FIG. 6A  illustrates an example of a position in a vertical direction of an upper end of a sheath in a state where an edge ring has worn down, and  FIG. 6B  illustrates an example of a corrected position in the vertical direction of the upper end of the sheath. 
         FIG. 7  illustrates another example of the edge ring. 
         FIG. 8  is a flowchart showing an etching method according to an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, various exemplary embodiments will be described. 
     In an exemplary embodiment, an apparatus for plasma processing is provided. The apparatus includes a chamber, a substrate support, a sheath adjuster, a radio frequency power source, a bias power source, and a controller. The substrate support has a lower electrode and an electrostatic chuck. The electrostatic chuck is provided on the lower electrode. The substrate support is configured to support a substrate which is placed thereon in the chamber. The sheath adjuster is configured to adjust a position in a vertical direction of an upper end of a sheath above an edge ring. The edge ring is disposed to surround an edge of the substrate. The radio frequency power source is configured to generate radio frequency power which is supplied to generate a plasma from a gas in the chamber. The radio frequency power has a first frequency. The bias power source is configured to generate bias power. The bias power source is electrically connected to the lower electrode. The bias power is set to vary a potential of the substrate placed on the electrostatic chuck within a cycle which is defined at a second frequency. The second frequency is lower than the first frequency. The controller is configured to control the sheath adjuster and the radio frequency power source. The controller is configured to control the radio frequency power source to supply the radio frequency power in at least a part of a first period in the cycle. In the first period, the potential of the substrate placed on the electrostatic chuck is higher than an average value of the potential of the substrate in the cycle. The controller is configured to control the radio frequency power source to reduce, in a second period in the cycle, a power level of the radio frequency power to be lower than a power level of the radio frequency power in the first period. In the second period, the potential of the substrate placed on the electrostatic chuck is lower than the average value of the potential of the substrate in the cycle. The controller is configured to control the sheath adjuster to adjust the position in the vertical direction of the upper end of the sheath in the first period and the second period. 
     If the position in the vertical direction of the upper end of the sheath is adjusted by the sheath adjuster, an impedance of a path of the radio frequency power that reaches the plasma via the edge ring increases. As a result, the power level of the radio frequency power which is supplied to the edge ring is lowered, and the power level of the radio frequency power which is supplied to the substrate relatively increases. Therefore, a difference is generated between an etching rate at the edge of the substrate and an etching rate of the substrate inside the edge. In the embodiment describe above, the radio frequency power is supplied in at least a part of the first period in the cycle of the bias power. In the first period, the potential difference between the substrate and the plasma is small, and thus the progress of etching of the substrate is relatively slow or etching of the substrate is not substantially performed. On the other hand, in the second period, the potential difference between the substrate and the plasma is large, and thus the etching of the substrate proceeds. However, the power level of the radio frequency power is set to a low level or zero. Therefore, according to the embodiment described above, even if the adjustment of the position in the vertical direction of the upper end of the sheath is performed by the sheath adjuster, the difference between the etching rate at the edge of the substrate and the etching rate of the substrate inside the edge becomes small. 
     In an exemplary embodiment, the controller may control the radio frequency power source to stop the supply of the radio frequency power in the second period. 
     In an exemplary embodiment, the sheath adjuster may be configured to apply a voltage to the edge ring to adjust the position in the vertical direction of the upper end of the sheath. 
     In an exemplary embodiment, the sheath adjuster may be configured to move the edge ring upward to adjust the position in the vertical direction of the upper end of the sheath. 
     In an exemplary embodiment, the bias power source may be configured to supply, as the bias power, radio frequency bias power having the second frequency to the lower electrode. 
     In an exemplary embodiment, the first period may be a period in which the radio frequency bias power which is output from the bias power source has a positive potential. The second period may be a period in which the radio frequency bias power which is output from the bias power source has a negative potential. 
     In an exemplary embodiment, the bias power source may be configured to apply, as the bias power, a pulsed direct-current voltage to the lower electrode at the cycle which is defined at the second frequency. 
     In an exemplary embodiment, the first period may be a period in which the pulsed direct-current voltage is not applied to the lower electrode. The second period may be a period in which the pulsed negative direct-current voltage having a negative polarity is applied to the lower electrode. 
     In an exemplary embodiment, the apparatus may further include a voltage sensor configured to measure the potential of the substrate. 
     In another exemplary embodiment, a method of etching in an apparatus for plasma processing is provided. The apparatus includes a chamber, a substrate support, a sheath adjuster, a radio frequency power source, and a bias power source. The substrate support has a lower electrode and an electrostatic chuck. The electrostatic chuck is provided on the lower electrode. The substrate support is configured to support a substrate which is placed thereon in the chamber. The sheath adjuster is configured to adjust a position in a vertical direction of an upper end of a sheath above an edge ring. The edge ring is disposed to surround an edge of the substrate. The radio frequency power source is configured to generate radio frequency power which is supplied to generate a plasma from a gas in the chamber. The radio frequency power has a first frequency. The bias power source is configured to generate bias power. The bias power source is electrically connected to the lower electrode. The bias power is set to vary a potential of the substrate placed on the electrostatic chuck within a cycle which is defined at a second frequency. The second frequency is lower than the first frequency. The method is performed in a state where the substrate is placed on the electrostatic chuck. The method includes supplying the radio frequency power in at least a part of a first period in the cycle. In the first period, the potential of the substrate is higher than an average value of the potential of the substrate in the cycle. The method further includes reducing, in a second period in the cycle, a power level of the radio frequency power to be lower than a power level of the radio frequency power in the first period. In the second period, the potential of the substrate is lower than the average value of the potential of the substrate in the cycle. Ions in the plasma generated in the chamber in the first period are accelerated toward the substrate in the second period, whereby the substrate placed on the electrostatic chuck is etched. In the first period and the second period, the position in the vertical direction of the upper end of the sheath is adjusted by the sheath adjuster. 
     In an exemplary embodiment, in the second period, supply of the radio frequency power may be stopped. 
     In an exemplary embodiment, the sheath adjuster may be configured to apply a voltage to the edge ring to adjust the position in the vertical direction of the upper end of the sheath. 
     In an exemplary embodiment, the sheath adjuster may be configured to move the edge ring upward to adjust the position in the vertical direction of the upper end of the sheath. 
     In an exemplary embodiment, the bias power source may be configured to supply, as the bias power, radio frequency bias power having the second frequency to the lower electrode. 
     In an exemplary embodiment, the first period may be a period in which the radio frequency bias power which is output from the bias power source has a positive potential. The second period may be a period in which the radio frequency bias power which is output from the bias power source has a negative potential. 
     In an exemplary embodiment, the bias power source may be configured to apply, as the bias power, a pulsed direct-current voltage to the lower electrode at the cycle which is defined at the second frequency. 
     In an exemplary embodiment, the first period may be a period in which the pulsed direct-current voltage is not applied to the lower electrode. The second period may be a period in which the pulsed negative direct-current voltage having a negative polarity is applied to the lower electrode. 
     Hereinafter, various exemplary embodiments will be described in detail with reference to the drawings. In the respective drawings, identical or equivalent parts are denoted by the same reference symbols. 
       FIG. 1  schematically illustrates a plasma processing apparatus according to an exemplary embodiment. An apparatus shown in  FIG. 1  (i.e. a plasma processing apparatus  1 ) is a capacitively coupled plasma processing apparatus. The plasma processing apparatus  1  is provided with a chamber  10 . The chamber  10  provides an internal space  10   s  therein. The central axis of the internal space  10   s  is an axis AX which extends in the vertical direction. In an embodiment, the chamber  10  includes a chamber body  12 . The chamber body  12  has a substantially cylindrical shape. The internal space  10   s  is provided in the chamber body  12 . The chamber body  12  is formed of, for example, aluminum. The chamber body  12  is electrically grounded. A film having plasma resistance is formed on the inner wall surface of the chamber body  12 , that is, the wall surface defining the internal space  10   s.  The film may be a film formed by anodization or a ceramic film such as a film formed of yttrium oxide. 
     A passage  12   p  is formed in a side wall of the chamber body  12 . A substrate W passes through the passage  12   p  when it is transferred between the internal space  10   s  and the outside of the chamber  10 . A gate valve  12   g  is provided along the side wall of the chamber body  12  for opening and closing of the passage  12   p.    
     The plasma processing apparatus  1  is further provided with a substrate support  16 . The substrate support  16  is configured to support the substrate W which is placed thereon in the chamber  10 . The substrate W has a substantially disk shape. The substrate support  16  is supported by the support  17 . The support  17  extends upward from a bottom portion of the chamber body  12 . The support  17  has a substantially cylindrical shape. The support  17  is formed of an insulating material such as quartz. 
     The substrate support  16  has a lower electrode  18  and an electrostatic chuck  20 . The lower electrode  18  and the electrostatic chuck  20  are provided in the chamber  10 . The lower electrode  18  is formed of a conductive material such as aluminum and has a substantially disk shape. 
     A flow path  18   f  is formed in the lower electrode  18 . The flow path  18   f  is a flow path for a heat exchange medium. As the heat exchange medium, a liquid refrigerant or a refrigerant (for example, chlorofluorocarbon) that cools the lower electrode  18  by vaporization thereof is used. A supply device of the heat exchange medium (for example, a chiller unit) is connected to the flow path  18   f.  The supply device is provided outside the chamber  10 . The heat exchange medium is supplied to the flow path  18   f  from the supply device  18   f  through a pipe  23   a.  The heat exchange medium supplied to the flow path  18   f  is returned to the supply device through a pipe  23   b.    
     The electrostatic chuck  20  is provided on the lower electrode  18 . The substrate W is placed on the electrostatic chuck  20  and held by the electrostatic chuck  20  when it is processed in the internal space  10   s.    
     The electrostatic chuck  20  has a main body and an electrode. The main body of the electrostatic chuck  20  is formed of a dielectric such as aluminum oxide or aluminum nitride. The main body of the electrostatic chuck  20  has a substantially disk shape. The central axis of the electrostatic chuck  20  substantially coincides with the axis AX. The electrode of the electrostatic chuck  20  is provided in the main body. The electrode of the electrostatic chuck  20  has a film shape. A direct-current power source is electrically connected to the electrode of the electrostatic chuck  20  through a switch. When the voltage from the direct-current power source is applied to the electrode of the electrostatic chuck  20 , an electrostatic attraction force is generated between the electrostatic chuck  20  and the substrate W. Due to the generated electrostatic attraction force, the substrate W is attracted to the electrostatic chuck  20  and held by the electrostatic chuck  20 . 
     The electrostatic chuck  20  includes a substrate placing region. The substrate placing region is a region having a substantially disk shape. The central axis of the substrate placing region substantially coincides with the axis AX. The substrate W is placed on the upper surface of the substrate placing region when it is processed in the chamber  10 . 
     In an exemplary embodiment, the electrostatic chuck  20  may further include an edge ring placing region. The edge ring placing region extends in a circumferential direction to surround the substrate placing region around the central axis of the electrostatic chuck  20 . An edge ring FR is mounted on the upper surface of the edge ring placing region. The edge ring FR has a ring shape. The edge ring FR is placed on the edge ring placing region such that the central axis thereof coincides with the axis AX. The substrate W is disposed in a region surrounded by the edge ring FR. That is, the edge ring FR is disposed to surround the edge of the substrate W. The edge ring FR may have electrical conductivity. The edge ring FR is formed of, for example, silicon or silicon carbide. The edge ring FR may be formed of a dielectric such as quartz. 
     The plasma processing apparatus  1  can be further provided with a gas supply line  25 . The gas supply line  25  supplies a heat transfer gas, for example, He gas, from a gas supply mechanism to a gap between the upper surface of the electrostatic chuck  20  and the rear surface (lower surface) of the substrate W. 
     The plasma processing apparatus  1  can be further provided with an insulating region  27 . The insulating region  27  is disposed on the support  17 . The insulating region  27  is disposed outside the lower electrode  18  in a radial direction with respect to the axis AX. The insulating region  27  extends in the circumferential direction along the outer peripheral surface of the lower electrode  18 . The insulating region  27  is formed of an insulator such as quartz. The edge ring FR is placed on the insulating region  27  and the edge ring placing region. 
     The plasma processing apparatus  1  is further provided with an upper electrode  30 . The upper electrode  30  is provided above the substrate support  16 . The upper electrode  30  closes an upper opening of the chamber body  12  together with a member  32 . The member  32  has insulation properties. The upper electrode  30  is supported on an upper portion of the chamber body  12  through the member  32 . 
     The upper electrode  30  includes a top plate  34  and a support  36 . The lower surface of the top plate  34  defines the internal space  10   s.  A plurality of gas discharge holes  34   a  are formed in the top plate  34 . Each of the plurality of gas discharge holes  34   a  penetrates the top plate  34  in a plate thickness direction (the vertical direction). Although being not limited, the top plate  34  is formed of silicon, for example. Alternatively, the top plate  34  can have a structure in which a plasma-resistant film is provided on the surface of a member made of aluminum. The film can be a film formed by anodization or a ceramic film such as a film formed of yttrium oxide. 
     The support  36  detachably supports the top plate  34 . The support  36  is formed of a conductive material such as aluminum, for example. A gas diffusion chamber  36   a  is provided in the interior of the support  36 . A plurality of gas holes  36   b  extend downward from the gas diffusion chamber  36   a.  The plurality of gas holes  36   b  communicate with the plurality of gas discharge holes  34   a,  respectively. A gas introduction port  36   c  is formed in the support  36 . The gas introduction port  36   c  is connected to the gas diffusion chamber  36   a.  A gas supply pipe  38  is connected to the gas introduction port  36   c.    
     A gas source group  40  is connected to the gas supply pipe  38  through a valve group  41 , a flow rate controller group  42 , and a valve group  43 . The gas source group  40 , the valve group  41 , the flow rate controller group  42 , and the valve group  43  configure a gas supply unit. The gas source group  40  includes a plurality of gas sources. Each of the valve group  41  and the valve group  43  includes a plurality of valves (for example, on-off valves). The flow rate controller group  42  includes a plurality of flow rate controllers. Each of the plurality of flow rate controllers of the flow rate controller group  42  is a mass flow controller or a pressure control type flow rate controller. Each of the plurality of gas sources of the gas source group  40  is connected to the gas supply pipe  38  through a corresponding valve of the valve group  41 , a corresponding flow rate controller of the flow rate controller group  42 , and a corresponding valve of the valve group  43 . The plasma processing apparatus  1  can supply one or more gases from one or more gas sources selected from the plurality of gas sources of the gas source group  40  to the internal space  10   s  at individually adjusted flow rates. 
     A baffle plate  48  is provided between the substrate support  16  or the support  17  and the side wall of the chamber body  12 . The baffle plate  48  can be configured, for example, by coating a member made of aluminum with ceramic such as yttrium oxide. A number of through-holes are formed in the baffle plate  48 . An exhaust pipe  52  is connected to the bottom portion of the chamber body  12  below the baffle plate  48 . An exhaust device  50  is connected to the exhaust pipe  52 . The exhaust device  50  includes a pressure controller such as an automatic pressure control valve, and a vacuum pump such as a turbo molecular pump, and is capable of reducing the pressure in the internal space  10   s.    
     The plasma processing apparatus  1  is further provided with a radio frequency power source  61 . The radio frequency power source  61  is a power source which generates radio frequency power HF. The radio frequency power HF is used to generate a plasma from the gas in the chamber  10 . The radio frequency power HF has a first frequency. The first frequency is a frequency in the range of 27 to 100 MHz, for example, a frequency of 40 MHz or 60 MHz. The radio frequency power source  61  is connected to the lower electrode  18  through a matching circuit  63  to supply the radio frequency power HF to the lower electrode  18 . The matching circuit  63  is configured to match the output impedance of the radio frequency power source  61  with the impedance on the load side (the lower electrode  18  side). The radio frequency power source  61  may not be electrically connected to the lower electrode  18  and may be connected to the upper electrode  30  through the matching circuit  63 . 
     The plasma processing apparatus  1  is further provided with a bias power source  62 . The bias power source  62  is electrically connected to the lower electrode  18 . The bias power source  62  generates bias power. The bias power is used to draw ions into the substrate W. The bias power is set to vary the potential of the substrate W placed on the electrostatic chuck  20  within a cycle P B  which is defined at a second frequency. The second frequency is lower than the first frequency. 
       FIG. 2  is a timing chart showing radio frequency power and bias power which are used in a plasma processing apparatus according to an exemplary embodiment. In an embodiment, as shown in  FIG. 2 , the bias power is radio frequency bias power LF. In this embodiment, the second frequency, that is, the frequency of the radio frequency bias power LF is a frequency in a range of 50 kHz to 27 MHz, and is, for example, 400 kHz. In this embodiment, the bias power source  62  is connected to the lower electrode  18  through a matching circuit  64  to supply the radio frequency bias power LF to the lower electrode  18 . The matching circuit  64  is configured to match the output impedance of the bias power source  62  with the impedance on the load side (the lower electrode  18  side). 
     In a case where plasma etching is performed in the plasma processing apparatus  1 , a gas is supplied to the internal space  10   s.  Then, the radio frequency power HF and the bias power are supplied, whereby the gas is excited in the internal space  10   s.  As a result, a plasma is generated in the internal space  10   s.  The substrate W is etched by chemical species such as ions and/or radicals from the generated plasma. That is, the plasma etching is performed. 
     Hereinafter,  FIGS. 3A and 3B  are referred to.  FIG. 3A  illustrates an example of a position in a vertical direction of an upper end of a sheath in a state where an edge ring has worn down.  FIG. 3B  illustrates an example of a corrected position in the vertical direction of the upper end of the sheath. In each of  FIG. 3A  and  FIG. 3B , the position in the vertical direction of the upper end (hereinafter referred to as an “upper end position”) of the sheath is indicated by a broken line. Further, in each of  FIG. 3A  and  FIG. 3B , a traveling direction of ions to the substrate W is indicated by an arrow. 
     If the plasma etching of the substrate W is performed, the edge ring FR wears down as shown in  FIG. 3A . If the edge ring FR has worn down, the thickness of the edge ring FR is reduced, so that the position in the vertical direction of the upper surface of the edge ring FR is lowered. If the position in the vertical direction of the upper surface of the edge ring FR is lowered, the upper end position of the sheath above the edge ring FR becomes lower than the upper end position of the sheath above the substrate W. As a result, the upper end of the sheath is inclined in the vicinity of the edge of the substrate W, and thus the traveling direction of ions which are supplied to the edge of the substrate W becomes a direction inclined with respect to the vertical direction. 
     In order to correct the traveling direction of ions to the vertical direction (that is, the direction perpendicular to the edge of the substrate W), the plasma processing apparatus  1  is further provided with a sheath adjuster  74 , as shown in  FIG. 1 . The sheath adjuster  74  is configured to adjust the upper end position of the sheath above the edge ring FR. The sheath adjuster  74  adjusts the upper end position of the sheath above the edge ring FR to eliminate or reduce the difference between the upper end position of the sheath above the edge ring FR and the upper end position of the sheath above the substrate W. 
     In an embodiment, the sheath adjuster  74  is a power source configured to apply a voltage V N  to the edge ring FR. The voltage V N  may have a negative polarity. In the embodiment, the sheath adjuster  74  is connected to the edge ring FR through a filter  75  and a conducting wire  76 . The filter  75  is a filter for blocking or reducing radio frequency power flowing into the sheath adjuster  74 . 
     The voltage V N  may be a direct-current voltage or a radio frequency voltage. The level of the voltage V N  determines the adjustment amount of the upper end position of the sheath. The adjustment amount of the upper end position of the sheath, that is, the level of the voltage V N  is determined according to a parameter that reflects the thickness of the edge ring FR. This parameter may be a measured value of the thickness of the edge ring FR which is measured optically or electrically, the position in the vertical direction of the upper surface of the edge ring FR which is measured optically or electrically, or the length of a time when the edge ring FR is exposed to the plasma. The level of the voltage V N  is determined using a predetermined relationship between such a parameter and the level of the voltage V N . For example, the predetermined relationship between the parameter and the level of the voltage V N  is determined in advance such that the absolute value of the voltage V N  increases as the thickness of the edge ring FR decreases. If the voltage V N  having the determined level is applied to the edge ring FR, as shown in  FIG. 3B , the difference between the upper end position of the sheath above the edge ring FR and the upper end position of the sheath above the substrate W is eliminated or reduced. 
     The voltage V N  may be a pulsed radio frequency voltage or a pulsed direct-current voltage. That is, the voltage V N  may be periodically applied to the edge ring FR. In a case where as the voltage V N , the pulsed direct-current voltage is periodically applied to the edge ring FR, the level of the voltage V N  may change during the period in which the voltage V N  is applied to the edge ring FR. 
     If the upper end position of the sheath is corrected by the sheath adjuster  74 , the impedance of the path of the radio frequency power which reaches the plasma via the edge ring increases. This is because the thickness of the sheath increases above the edge ring FR if the upper end position of the sheath is adjusted by applying the voltage V N  to the edge ring FR. If the impedance of the path of the radio frequency power which reaches the plasma via the edge ring increases, the power level of the radio frequency power which is supplied to the edge ring is lowered. Further, if the impedance of the path of the radio frequency power which reaches the plasma via the edge ring increases, the power level of the radio frequency power which is supplied to the substrate W increases. As a result, the etching rate of the substrate W inside the edge becomes higher than the etching rate at the edge of the substrate W. 
     In the plasma processing apparatus  1 , a supply timing of the radio frequency power HF is controlled by a controller MC to reduce the difference between the etching rate at the edge of the substrate and the etching rate of the substrate inside the edge. 
     The controller MC is a computer which includes a processor, a storage device, an input device, a display device, and the like, and controls each part of the plasma processing apparatus  1 . The controller MC executes a control program stored in the storage device and controls each part of the plasma processing apparatus  1 , based on recipe data stored in the storage device. A process designated by the recipe data is executed in the plasma processing apparatus  1  by the control by the controller MC. A method of etching according to an embodiment to be described later can be executed in the plasma processing apparatus  1  by the control of each part of the plasma processing apparatus  1  by the controller MC. 
     The controller MC can determine the level of the voltage V N , as described above. The predetermined relationship between the parameter described above and the level of the voltage V N  may be stored in the storage device of the controller MC as a function or table format data. The controller MC can control the sheath adjuster  74  to apply the voltage V N  having the determined level to the edge ring FR. 
     The controller MC controls the radio frequency power source  61  to supply the radio frequency power HF in at least a part of a first period P 1  in the cycle P B . The radio frequency power HF may be supplied in the whole of the first period P 1 . In the first period P 1 , the potential of the substrate W placed on the electrostatic chuck  20  is higher than the average value V AVE  of the potential of the substrate W in the cycle P B . The controller MC controls the radio frequency power source  61  to reduce, in a second period P 2  in the cycle P B , the power level of the radio frequency power HF to be lower than the power level of the radio frequency power HF in the first period P 1 . In the second period P 2 , the potential of the substrate W placed on the electrostatic chuck  20  is lower than the average value V AVE . In an embodiment, the controller MC may control the radio frequency power source  61  to stop supply of the radio frequency power HF in the second period P 2 . The controller MC controls the sheath adjuster  74  to adjust the position in the vertical direction of the upper end of the sheath in the first period P 1  and the second period P 2 . 
     In the plasma processing apparatus  1 , the radio frequency power HF is supplied in at least a part of the first period P 1  in the cycle P B  of the bias power. In the first period P 1 , the potential difference between the substrate W and the plasma is small, and thus the progress of the etching of the substrate W is relatively slow, or the etching of the substrate W is not substantially performed. On the other hand, in the second period P 2 , the potential difference between the substrate W and the plasma is large and the etching of the substrate W proceeds. However, the power level of the radio frequency power HF is set to a low level or zero. Therefore, according to the plasma processing apparatus  1 , even if the adjustment of the position in the vertical direction of the upper end of the sheath is performed by the sheath adjuster  74 , the difference between the etching rate at the edge of the substrate W and the etching rate of the substrate inside the edge becomes small. 
     In an embodiment, as shown in  FIG. 2 , the radio frequency bias power LF is used as the bias power. The first period P 1  may be a period in which the radio frequency bias power LF which is output from the bias power source  62  has a positive potential. The second period P 2  may be a period in which the radio frequency bias power LF which is output from the bias power source  62  has a negative potential. In this embodiment, a synchronization pulse, a delay time length, and a supply time length are provided from the controller MC to the radio frequency power source  61 . The synchronization pulse is synchronized with the radio frequency bias power LF. The delay time length is a delay time length from the point in time of the start of the cycle P B  which is specified by the synchronization pulse. The supply time length is the length of a supply time of the radio frequency power HF. The radio frequency power source  61  supplies the radio frequency power HF to the lower electrode  18  during the supply time length from a point in time delayed by the delay time length with respect to the point in time of the start of the cycle P B . As a result, the radio frequency power HF is supplied to the lower electrode  18  in the first period P 1 . The delay time length may be zero. 
     In an embodiment, the plasma processing apparatus  1  may be further provided with a voltage sensor  78 . The voltage sensor  78  is configured to directly or indirectly measure the potential of the substrate W. In the example shown in  FIG. 1 , the voltage sensor  78  is configured to measure the potential of the lower electrode  18 . Specifically, the voltage sensor  78  measures the potential of the power supply path connected between the lower electrode  18  and the bias power source  62 . In this embodiment, the controller MC determines a period in which the potential of the substrate W measured by the voltage sensor  78  is higher than the average value V AVE  of the potential of the substrate W in the cycle P B  as the first period P 1 . The controller MC controls the radio frequency power source  61  to supply the radio frequency power HF in the first period P 1 . The controller MC determines a period in which the potential of the substrate W measured by the voltage sensor  78  is lower than the average value V AVE  as the second period P 2 . The controller MC controls the radio frequency power source  61  to reduce the power level of the radio frequency power HF in the second period P 2  to be lower than the power level of the radio frequency power HF in the first period P 1 , or stop the supply of the radio frequency power HF. The average value V AVE  of the potential of the substrate W may be a value determined in advance. 
     Hereinafter, another embodiment will be described.  FIG. 4  is a timing chart showing radio frequency power and bias power which are used in a plasma processing apparatus according to another exemplary embodiment. In the plasma processing apparatus  1  according to another exemplary embodiment, the bias power source  62  is configured to apply a pulsed direct-current voltage VB as the bias power to the lower electrode  18 . The direct-current voltage VB is applied to the lower electrode  18  at a repetition frequency that is the second frequency. In an embodiment in which the pulsed direct-current voltage VB is used, the second frequency is 50 kHz or more and 27 MHz or less. In this embodiment, the matching circuit  64  can be omitted. In this embodiment, the first period P 1  may be a period in which the direct-current voltage VB is not applied to the lower electrode. The second period P 2  may be a period in which the pulsed direct-current voltage VB having a negative polarity is applied to the lower electrode. Alternatively, as described above, the first period P 1  and the second period P 2  may be determined from the potential of the substrate W measured by the voltage sensor  78 . In other respects, the plasma processing apparatus  1  that uses the direct-current voltage VB as the bias power may be the same as the plasma processing apparatus  1  that uses the radio frequency bias power LF as the bias power. 
     Hereinafter, a plasma processing apparatus according to a still another embodiment will be described with reference to  FIGS. 5, 6A , and  6 B.  FIG. 5  schematically illustrates the plasma processing apparatus according to still another exemplary embodiment.  FIG. 6A  illustrates an example of a position in a vertical direction of an upper end of the sheath in a state where an edge ring has worn down, and  FIG. 6B  illustrates an example of a corrected position in the vertical direction of the upper end of the sheath. In each of  FIG. 6A  and  FIG. 6B , the upper end position of the sheath is indicated by a broken line. Further, in each of  FIG. 6A  and  FIG. 6B , a traveling direction of ions to the substrate W is indicated by an arrow. 
     A plasma processing apparatus  1 B shown in  FIG. 5  is different from the plasma processing apparatus  1  in that the plasma processing apparatus  1 B uses an edge ring FRB instead of the edge ring FR. Further, the plasma processing apparatus  1 B is different from the plasma processing apparatus  1  in that the plasma processing apparatus  1 B is provided with a sheath adjuster  74 B instead of the sheath adjuster  74 . In other respects, the configuration of the plasma processing apparatus  1 B may be the same as the configuration of the plasma processing apparatus  1 . 
     As shown in  FIGS. 6A and 6B , the edge ring FRB has a first annular part FR 1  and a second annular part FR 2 . The first annular part FR 1  and the second annular part FR 2  are separated from each other. The first annular part FR 1  has an annular plate shape and is placed on the edge ring placing region to extend around the axis AX. The substrate W is placed on the substrate placing region such that the edge thereof is located on/above the first annular part FR 1 . The second annular part FR 2  has an annular plate shape and is placed on the edge ring placing region to extend around the axis AX. The second annular part FR 2  is located outside the first annular part FR 1  in the radial direction. 
     The sheath adjuster  74 B is a movement device configured to move the edge ring FRB upward to adjust the position in the vertical direction of the upper surface of the edge ring FRB. Specifically, the sheath adjuster  74 B is configured to move the second annular part FR 2  upward to adjust the position in the vertical direction of the upper surface of the second annular part FR 2 . In an example, the sheath adjuster  74 B includes a drive device  74   a  and a shaft  74   b.  The shaft  74   b  supports the second annular part FR 2  and extends downward from the second annular part FR 2 . The drive device  74   a  is configured to generate a driving force for moving the second annular part FR 2  in the vertical direction through the shaft  74   b.    
     If the plasma etching of the substrate W is performed, the edge ring FRB wears down as shown in  FIG. 6A . If the edge ring FRB has worn down, the thickness of the second annular part FR 2  decreases, and thus the position in the vertical direction of the upper surface of the second annular part FR 2  is lowered. If the position in the vertical direction of the upper surface of the second annular part FR 2  is lowered, the upper end position of the sheath on/above the edge ring FRB becomes lower than the upper end position of the sheath above the substrate W. As a result, the upper end of the sheath is inclined in the vicinity of the edge of the substrate W, so that the traveling direction of ions which are supplied to the edge of the substrate W becomes a direction inclined with respect to the vertical direction. 
     In order to correct the traveling direction of ions to the vertical direction, the sheath adjuster  74 B is configured to adjust the upper end position of the sheath on/above the edge ring FRB. The sheath adjuster  74 B adjusts the upper end position of the sheath on/above the edge ring FRB to eliminate or reduce the difference between the upper end position of the sheath on/above the edge ring FRB and the upper end position of the sheath above the substrate W. Specifically, the sheath adjuster  74 B moves the second annular part FR 2  upward to make the position in the vertical direction of the upper surface of the second annular part FR 2  coincide with the position in the vertical direction of the upper surface of the substrate W on the electrostatic chuck  20 . 
     The adjustment amount of the upper end position of the sheath, that is, the amount of movement of the second annular part FR 2  is determined according to a parameter which reflects the thickness of the edge ring FRB, that is, the thickness of the second annular part FR 2 . This parameter may be the measured value of the thickness of the second annular part FR 2  which is measured optically or electrically, the position in the vertical direction of the upper surface of the second annular part FR 2  which is measured optically or electrically, or the length of a time when the edge ring FRB is exposed to the plasma. The amount of movement of the second annular part FR 2  is determined using a predetermined relationship between such a parameter and the amount of movement of the second annular part FR 2 . For example, the predetermined relationship between the parameter and the amount of movement of the second annular part FR 2  is determined in advance such that the amount of movement of the second annular part FR 2  increases as the thickness of the second annular part FR 2  decreases. If the second annular part FR 2  is moved upward by the determined amount of movement, as shown in  FIG. 6B , the difference between the upper end position of the sheath on/above the edge ring FRB and the upper end position of the sheath above the substrate W is eliminated or reduced. 
     In the plasma processing apparatus  1 B, the controller MC can determine the amount of movement of the second annular part FR 2 , as described above. The predetermined relationship between the parameter described above and the amount of movement of the second annular part FR 2  may be stored in the storage device of the controller MC as a function or table format data. The controller MC can control the sheath adjuster  74 B to move the second annular part FR 2  upward by the determined amount of movement. 
     If the upper end position of the sheath is corrected by the sheath adjuster  74 B, the impedance of the path of the radio frequency power which reaches the plasma via the edge ring FRB increases. This is because the gap immediately below the second annular part FR 2  is widened. If the impedance of the path of the radio frequency power which reaches the plasma via the edge ring FRB increases, the power level of the radio frequency power which is supplied to the edge ring FRB decreases. Further, if the impedance of the path of the radio frequency power which reaches the plasma via the edge ring FRB increases, the power level of the radio frequency power which is supplied to the substrate W increases relatively. As a result, the etching rate of the substrate W inside the edge becomes higher than the etching rate at the edge of the substrate W. Therefore, in the plasma processing apparatus  1 B, similar to the plasma processing apparatus  1 , the supply timing of the radio frequency power HF is controlled by the controller MC. 
       FIG. 7  is a diagram showing another example of the edge ring. In the edge ring FRB shown in  FIG. 7 , the first annular part FR 1  has an inner peripheral portion and an outer peripheral portion. The position in the vertical direction of the upper surface of the inner peripheral portion is lower than the position in the height direction in the vertical direction of the upper surface of the outer peripheral portion. The substrate W is placed on the substrate placing region such that the edge thereof is located on or above the inner peripheral portion of the first annular part FR 1 . The second annular part FR 2  is disposed on the inner peripheral portion of the first annular part FR 1  to surround the edge of the substrate W. That is, in the edge ring FRB shown in  FIG. 7 , the second annular part FR 2  is disposed inside the outer peripheral portion of the first annular part FR 1 . In a case where the edge ring FRB shown in  FIG. 7  is used, the shaft  74   b  of the sheath adjuster  74 B extends downward from the second annular part FR 2  to penetrate the inner peripheral portion of the first annular part FR 1 . 
     Hereinafter,  FIG. 8  is referred to.  FIG. 8  is a flowchart showing an etching method according to an exemplary embodiment. A method shown in  FIG. 8  (i.e. an etching method MT) is performed using any one of plasma processing apparatuses according to various embodiments, such as the plasma processing apparatus  1  and the plasma processing apparatus  1 B described above. 
     The etching method MT is performed in a state where the substrate W is placed on the electrostatic chuck  20 . In the etching method MT, a gas is supplied from the gas supply unit into the chamber  10 . Then, the pressure in the chamber  10  is set to a designated pressure by the exhaust device  50 . Then, in the etching method MT, as the bias power described above, the radio frequency bias power LF or the pulsed direct-current voltage VB is supplied to the lower electrode  18 . 
     In the etching method MT, step ST 1  is executed. Step ST 1  is executed in at least a part of the first period P 1  in the cycle P B  of the bias power. In step ST 1 , the radio frequency power HF is supplied. Subsequent step ST 2  is executed in the second period P 2  in the cycle P B  of the bias power. In step ST 2 , the power level of the radio frequency power HF is reduced to be lower than the power level of the radio frequency power HF in the first period P 1 , or the supply of the radio frequency power HF is stopped. In the etching method MT, ions in the plasma generated in the chamber  10  in the first period P 1  are accelerated toward the substrate W in the second period P 2 , whereby the substrate W is etched. In the etching method MT, the position in the vertical direction of the upper end of the sheath is adjusted by the sheath adjuster (the sheath adjuster  74  or the sheath adjuster  74 B) in the first period P 1  and the second period P 2 . 
     In step ST 3 , whether a stop condition is satisfied or not is determined. The stop condition is satisfied in a case where the number of repetitions of step ST 1  and step ST 2  has reached a predetermined number of times. In step ST 3 , if it is determined that the stop condition is not satisfied, step ST 1  and step ST 2  are executed again. On the other hand, in step ST 3 , if it is determined that the stop condition is satisfied, the execution of the etching method MT is ended. 
     While various exemplary embodiments have been described above, various additions, omissions, substitutions and changes may be made without being limited to the exemplary embodiments described above. Elements of the different embodiments may be combined to form another embodiment. 
     The plasma processing apparatus according to another embodiment may be a capacitively coupled plasma processing apparatus different from the plasma processing apparatus  1  and the plasma processing apparatus  1 B. Further, the plasma processing apparatus according to still another embodiment may be an inductively coupled plasma processing apparatus. Further, the plasma processing apparatus according to still another embodiment may be an ECR (electron cyclotron resonance) plasma processing apparatus. Further, the plasma processing apparatus according to still another embodiment may be a plasma processing apparatus that generates plasma by using surface waves such as microwaves. 
     From the foregoing description, it will be appreciated that various embodiments of the present disclosure have been described herein for purposes of illustration, and that various modifications may be made without departing from the scope and spirit of the present disclosure. Accordingly, the various embodiments disclosed herein are not intended to be limiting, with the true scope and spirit being indicated by the following claims.