Patent Publication Number: US-2022223427-A1

Title: Plasma processing apparatus and system

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 16/674,461 filed on Nov. 5, 2019, which claims priority from Japanese Patent Application Nos. 2018-208005 and 2019-185832, filed on Nov. 5, 2018 and Oct. 9, 2019, respectively, with the Japan Patent Office, all of which are incorporated herein in their entirety by reference and priority is claimed to each. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to an etching method and a plasma processing apparatus. 
     BACKGROUND 
     A plasma processing apparatus used in a plasma etching of a substrate includes a chamber and a substrate support. The substrate support includes a lower electrode and is provided in the chamber. Plasma is generated from the gas in the chamber where the substrate is placed on the substrate support for etching. The substrate is etched by the positive ions from the plasma to form openings in the substrate. 
     The substrate is charged as the etching of the substrate with the positive ions progressed. In a state where the substrate is charged, the amount of positive ions supplied into the openings decreases. As a result, the etching rate may be reduced. A shape abnormality may also occur in the openings formed in the substrate. 
     In the technique described in Japanese Patent Laid-Open Publication No. 2012-079886, a positive DC voltage is applied from a power source to a lower electrode in order to reduce the positive charge amount of a substrate. Subsequently, the application of the DC voltage to the lower electrode is stopped. Subsequently, a negative DC voltage is applied from a power supply to the lower electrode. As a result, positive ions are drawn into the substrate and etching is performed. Thereafter, the application of the DC voltage to the lower electrode is stopped. In the technique described in Japanese Patent Laid-Open Publication No. 2012-079886, the application of the positive DC voltage to the lower electrode, the stop of the application of the DC voltage to the lower electrode, the application of the negative DC voltage to the lower electrode, and the stop of the application of the DC voltage to the lower electrode are repeated. The radio-frequency power that generates plasma is continuously supplied when the application of the positive DC voltage to the lower electrode, the stop of the application of the DC voltage to the lower electrode, the application of the negative DC voltage to the lower electrode, and the stop of the application of the DC voltage to the lower electrode are repeated. 
     SUMMARY 
     In an embodiment, an etching method performed in a plasma processing apparatus is provided. The etching method is performed in a state in which a substrate is placed on a substrate support provided in a chamber of the plasma processing apparatus. The etching method includes supplying radio-frequency power to generate plasma from a gas in the chamber. The etching method further includes applying a negative DC voltage to a lower electrode of the substrate support during execution of the supply of radio-frequency power in order to etch the substrate with positive ions from plasma. The etching method further includes stopping the application of the negative DC voltage to the lower electrode and the supply of the radio-frequency power in order to generate negative ions. The etching method further includes applying a positive DC voltage to the lower electrode in a state where the supply of radio-frequency power is stopped in order to supply negative ions to the substrate. 
     The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features will become apparent by reference to the drawings and the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a flowchart illustrating an etching method according to an embodiment. 
         FIG. 2  is a diagram schematically illustrating a plasma processing apparatus according to an embodiment. 
         FIG. 3  is a timing chart of an example related to the etching method illustrated in  FIG. 1 . 
         FIG. 4A  is a diagram illustrating the state of plasma and a substrate in a period P 1  in the timing chart of  FIG. 3 , and  FIG. 4B  is a diagram illustrating the state plasma and a substrate in a period P 2  in the timing chart of  FIG. 3 . 
         FIG. 5A  is a diagram illustrating the state of plasma and a substrate in a period P 31  in the timing chart of  FIG. 3 , and  FIG. 5B  is a diagram illustrating the state plasma and a substrate in a period P 32  in the timing chart of  FIG. 3 . 
         FIG. 6A  is a diagram illustrating the state of plasma and a substrate in a period P 4  in the timing chart of  FIG. 3 , and  FIG. 6B  is a diagram for explaining step ST 5  of the etching method illustrated in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to the accompanying drawings, which form a part thereof. The illustrative exemplary embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other exemplary embodiments may be utilized, and other changes may be made without departing from the spirit or scope of the subject matter presented here. 
     Various exemplary embodiments will be described below. 
     In an embodiment, an etching method performed using a plasma processing apparatus is provided. The etching method is performed in a state in which a substrate is placed on a substrate support provided in the chamber of the plasma processing apparatus. The etching method includes supplying radio-frequency power to generate plasma from a gas in the chamber. The etching method further includes applying a negative DC voltage to a lower electrode of the substrate support during the execution of the supply of radio-frequency power in order to etch the substrate with positive ions from plasma. The etching method further includes stopping the application of the negative DC voltage to the lower electrode and the supply of the radio-frequency power in order to generate negative ions. The etching method further includes applying a positive DC voltage to the lower electrode in a state where the supply of radio-frequency power is stopped in order to supply negative ions to the substrate. 
     In the above embodiment, a negative DC voltage is supplied to the lower electrode in a state where plasma is generated by the supply of radio-frequency power. As a result, positive ions collide with the substrate so as to etch the substrate. Subsequently, the supply of the radio-frequency power and the application of the DC voltage to the lower electrode are stopped. Although the amount of negative ions generated is small in a state where radio-frequency power is supplied, negative ions are efficiently generated by electrons which are attached to chemical species in the gas when the supply of radio-frequency power is stopped. Subsequently, a positive DC voltage is applied to the lower electrode in a state where the supply of radio-frequency power is stopped. As a result, negative ions are supplied to the substrate. According to the above embodiment, the negative ions decrease the positive charge amount of the substrate. Also, the substrate is etched using both positive ions and negative ions. Therefore, the etching efficiency is improved. 
     In an embodiment, the etching method may further include exhausting a gas from the interior space of the chamber. The discharging step is executed after one or more executions of an etching sequence including supplying radio-frequency power, applying a negative DC voltage, stopping, and applying a positive DC voltage. At the time of executing the discharging step, the supply of the radio-frequency power is stopped and the application of the DC voltage to the lower electrode is stopped. 
     In an embodiment, another sequence may be repeated that includes one or more executions of the etching sequence and the discharging step. 
     In an embodiment, the discharging step may be executed for 10 μsec or more in the execution period of the other sequence. In the embodiment, etching byproducts are more reliably discharged. As a result, the etching efficiency of the substrate is further improved. 
     In an embodiment, the time length of the execution period of the discharging step may be increased as the number of executions of the other sequence increases. In the embodiment, as the depth of the opening formed in the substrate increases, the time length of the execution period of the discharging step is increased. Therefore, etching byproducts are more reliably discharged. 
     In an embodiment, a parameter representing an electron density in the chamber may be measured during execution of the stopping step. The step of applying the positive DC voltage may be started when it is determined from the parameter that the electron density in the chamber is decreased to satisfy a predetermined standard. The decrease in electron density during the stopping step reflects an increase in the amount of negative ions. Therefore, according to the embodiment, the step of applying the positive DC voltage is started when it is determined that the negative ions are sufficiently generated. 
     In an embodiment, in the stopping step, the application of the negative DC voltage to the lower electrode may be stopped before the supply of the radio-frequency power is stopped. According to the embodiment, an abnormal discharge is more reliably prevented. 
     In another embodiment, a plasma processing apparatus is provided. The plasma processing apparatus includes a chamber, a substrate support, a radio-frequency power source, a power supply unit, and a controller. The substrate support has a lower electrode and is provided in the chamber. The radio-frequency power source is configured to supply radio-frequency power to generate plasma from a gas in the chamber. The power supply unit is configured to generate a positive DC voltage and a negative DC voltage. The power supply unit is electrically connected to the lower electrode. The controller is configured to control the radio-frequency power source and the power supply unit. The controller is configured to execute a first control, a second control, a third control, and a fourth control. The first control includes controlling the radio-frequency power source to supply radio-frequency power to generate plasma from the gas in the chamber. The second control includes controlling the power supply unit so as to apply a negative DC voltage to the lower electrode of the substrate support during the supply of the radio-frequency power in order to etch the substrate with positive ions from the plasma. The third control includes controlling the power supply unit and the radio-frequency power source to stop the application of the negative DC voltage to the lower electrode and the supply of the radio-frequency power in order to generate negative ions. The fourth control includes controlling the power supply unit so as to apply a positive DC voltage to the lower electrode in a state where the supply of radio-frequency power is stopped in order to supply negative ions to the substrate. 
     In an embodiment, the plasma processing apparatus may further include an exhaust device connected to the chamber. The controller may be configured to further execute a fifth control. The fifth control includes controlling the exhaust device to exhaust a gas from the internal space of the chamber. The fifth control is executed after one or more executions of the etching control sequence including the first control, the second control, the third control, and the fourth control. When the fifth control is executed, the supply of radio-frequency power is stopped, and the application of the DC voltage to the lower electrode is stopped. 
     In an embodiment, the controller may repeatedly execute another control sequence including one or more executions of the etching control sequence and the fifth control. 
     In an embodiment, the fifth control may be executed for 10 μsec or more in the execution period of the other control sequence. 
     In an embodiment, the controller may increase the time length of the execution period of the fifth control as the number of executions of the other control sequence increases. 
     In an embodiment, the plasma processing apparatus may further include a measuring device. The measuring device measures a parameter representing the electron density in the chamber during execution of the third control. The controller may start the execution of the fourth control when it is determined from the parameter that the electron density in the chamber is decreased to satisfy a predetermined standard. 
     In an embodiment, in the third control, the controller may control the power supply unit to stop applying the negative DC voltage to the lower electrode before stopping the supply of the radio-frequency power. 
     Various embodiments will now be described in detail with reference to the drawings. Further, in the respective drawings, the same or corresponding parts will be denoted by the same symbols. 
       FIG. 1  is a flowchart illustrating an etching method according to an embodiment. The etching method illustrated in  FIG. 1  (hereinafter, referred to as a “method MT”) is performed using a plasma processing apparatus.  FIG. 2  is a diagram schematically illustrating a plasma processing apparatus according to an embodiment. The plasma processing apparatus  1  illustrated in  FIG. 2  may be used to execute the method MT. 
     The plasma processing apparatus  1  is a capacitively coupled plasma processing apparatus. The plasma processing apparatus  1  includes a chamber  10 . The chamber  10  provides an internal space  10   s  therein. In the 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 made of, for example, aluminum. The chamber body  12  is electrically grounded. A plasma-resistant film is formed on the inner wall surface of the chamber body  12 , that is, the wall surface that defines the internal space  10   s . The film may be a ceramic film such as a film formed by anodization or a film formed from yttrium oxide. 
     A passage  12   p  is formed in the side wall of the chamber body  12 . A substrate W passes through the passage  12   p  when being 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  so as to open and close the passage  12   p.    
     A substrate support, that is, a support stage  16  is provided in the chamber  10 . The support stage  16  is configured to support the substrate W placed thereon. The substrate W has a substantially disk shape. The support stage  16  is supported by a support body  15 . The support body  15  extends upward from the bottom of the chamber body  12 . The chamber body  15  has a substantially cylindrical shape. The support body  15  is made of an insulating material such as quartz. 
     The support stage  16  has a lower electrode  18 . The support stage  16  may further include an electrostatic chuck  20 . The support stage  16  may further include an electrode plate  19 . The electrode plate  19  is made of a conductive material such as aluminum and has a substantially disk shape. The lower electrode  18  is provided on the electrode plate  19 . The lower electrode  18  is made of a conductive material such as aluminum and has a substantially disk shape. The lower electrode  18  is electrically connected to the electrode plate  19 . 
     A flow path  18   f  is formed in the lower electrode  18 . The flow path  18   f  is a flow path for the heat exchange medium. As for the heat exchange medium, a liquid refrigerant or a refrigerant that cools the lower electrode  18  by vaporization thereof (e.g., chlorofluorocarbon) is used. A circulation device of the heat exchange medium (e.g., a chiller unit) is connected to the flow path  18   f . The circulation device is provided outside the chamber  10 . A heat exchange medium is supplied to the flow path  18   f  from the circulation device through a pipe  23   a . The heat exchange medium supplied to the flow path  18   f  is returned to the circulation device through a pipe  23   b.    
     The electrostatic chuck  20  is provided on the lower electrode  18 . When the substrate W is processed in the internal space  10   s , the substrate W is placed on the electrostatic chuck  20  and held by the electrostatic chuck  20 . The electrostatic chuck  20  has a main body and electrodes. The main body of the electrostatic chuck  20  is made of a dielectric such as aluminum oxide or aluminum nitride. The main body of the electrostatic chuck  20  has a substantially disk shape. The electrostatic chuck  20  includes a substrate placement region and a focus ring mounting region. The substrate placement region is a region having a substantially disk shape. The upper surface of the substrate placement region extends along a horizontal plane. An axis AX that includes the center of the substrate placement region and extends in the vertical direction substantially coincides with the center axis of the chamber  10 . The substrate W is placed on the upper surface of the substrate placement region when processed in the chamber  10 . 
     The focus ring mounting region extends in the circumferential direction to surround the substrate mounting region. A focus ring FR is mounted on the upper surface of the focus ring mounting region. The focus ring FR has a ring shape. The substrate W is disposed in a region surrounded by the focus ring FR. That is, the focus ring FR surrounds the edge of the substrate W placed on the substrate placement region of the electrostatic chuck  20 . The focus ring FR is made of, for example, silicon or silicon carbide. 
     The electrode of the electrostatic chuck  20  is provided in the main body of the electrostatic chuck  20 . The electrode of the electrostatic chuck  20  is a film formed from a conductor. A DC power supply is electrically connected to the electrode of the electrostatic chuck  20 . When a DC voltage is applied from a DC power source to the electrode of the electrostatic chuck  20 , an electrostatic attraction is generated between the electrostatic chuck  20  and the substrate W. Due to the generated electrostatic attraction, the substrate W is attracted to the electrostatic chuck  20  and held by the electrostatic chuck  20 . 
     The plasma processing apparatus  1  may further include a gas supply line  25 . The gas supply line  25  supplies a heat transfer gas from a gas supply mechanism, for example, He gas, between the upper surface of the electrostatic chuck  20  and the back surface (lower surface) of the substrate W. 
     The plasma processing apparatus  1  may further include a tubular portion  28  and an insulating portion  29 . The tubular portion  28  extends upward from the bottom of the chamber body  12 . The tubular portion  28  extends along the outer periphery of the support body  15 . The tubular portion  28  is made of a conductive material and has a substantially cylindrical shape. The tubular portion  28  is electrically grounded. The insulating portion  29  is provided on the tubular portion  28 . The insulating portion  29  is made of an insulating material. The insulating portion  29  is made of ceramic such as, for example, quartz. The insulating portion  29  has a substantially cylindrical shape. The insulating portion  29  extends along the outer periphery of the electrode plate  19 , the outer periphery of the lower electrode  18 , and the outer periphery of the electrostatic chuck  20 . 
     The plasma processing apparatus  1  further includes an upper electrode  30 . The upper electrode  30  is provided above the support stage  16 . The upper electrode  30  closes an upper opening of the chamber body  12 . The upper electrode  30  is supported on the upper portion of the chamber body  12 . 
     The upper electrode  30  includes a top plate  34  and a support body  36 . The lower surface of the top plate  34  defines an 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 the plate thickness direction (vertical direction). Although the top plate  34  is not limited, the top plate  34  is formed from, for example, silicon. Alternatively, the top plate  34  may have a structure in which a plasma-resistant film is provided on the surface of an aluminum member. The film may be a ceramic film such as a film formed by anodization or a film formed from yttrium oxide. 
     The support body  36  detachably supports the top plate  34 . The support body  36  is made of a conductive material such as, for example, aluminum. A gas diffusion chamber  36   a  is provided inside the support body  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 body  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  via 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  constitute a gas supply. The gas source group  40  includes a plurality of gas sources. The plurality of gas sources include one or more gas sources used in etching methods according to various embodiments. Each of the valve group  41  and the valve group  43  includes a plurality of valves (e.g., open/close valves). The flow rate controller group  42  includes a plurality of flow rate controllers. Each of the plurality of flow controllers in the flow controller group  42  is a mass flow controller or a pressure control type flow controller. Each of the plurality of gas sources of the gas source group  40  is connected to the gas supply pipe  38  via 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  may supply the gas 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 member  48  is provided between the tubular portion  28  and the side wall of the chamber body  12 . The baffle member  48  may be a plate-like member. The baffle member  48  may be configured by coating, for example, a plate made of aluminum with a ceramic such as yttrium oxide. The baffle member  48  has a plurality of through holes. Below the baffle member  48 , an exhaust pipe  52  is connected to the bottom of the chamber body  12 . 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 may reduce the pressure in the internal space  10   s.    
     The plasma processing apparatus  1  further includes a radio-frequency power source  61 . The radio-frequency power source  61  is a power source that generates radio-frequency power for plasma generation. The frequency of the radio-frequency power is not limited, but is a frequency within a range of 27 to 100 MHz, for example, 40 MHz or 60 MHz The radio-frequency power source  61  is connected to the lower electrode  18  through a matcher  63  and the electrode plate  19  in order to supply radio-frequency power to the lower electrode  18 . The matcher  63  has a matching circuit that matches the output impedance of the radio-frequency power source  61  with the impedance on the load side (lower electrode  18  side). Further, 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  via the matcher  63 . 
     The plasma processing apparatus  1  further includes a power supply unit  64 . The power supply unit  64  is configured to generate a DC voltage applied to the lower electrode  18 . The power supply unit  64  is configured to generate a negative DC voltage and a positive DC voltage. The power supply unit  64  is electrically connected to the lower electrode  18 . In an embodiment, the power supply unit  64  is connected to an electrical path that connects the matcher  63  and the electrode plate  19  to each other via a low-path filter  66 . 
     In the plasma processing apparatus  1 , a gas is supplied to the internal space  10   s . In addition, radio-frequency power is supplied to excite gas in the internal space  10   s . As a result, plasma is generated in the internal space  10   s . The substrate W is processed by chemical species such as ions and/or radicals from the generated plasma. 
     In an embodiment, the plasma processing apparatus  1  may further include a measuring device  70 . The measuring device  70  is configured to measure a parameter representing the electron density in the chamber  10 . In an example, the measuring device  70  is a plasma absorption probe. In the example, the measuring device  70  includes a network analyzer  70   a , a high-pass filter  70   f , and a probe  70   p . The probe  70   p  extends from the outside of the chamber  10  to the inside of the chamber  10 . The network analyzer  70   a  is connected to the probe  70   p  through the high-pass filter  70   f . The network analyzer  70   a  supplies a weak power electromagnetic wave signal to the probe  70   p  while changing its frequency, and acquires an S11 parameter from the reflected signal returned from the probe  70   p . The network analyzer  70   a  specifies the electron density in the chamber  10  from the frequency corresponding to the minimum peak of the S11 parameter in the frequency characteristic of the S11 parameter. The specified electron density is used by a controller MC (to be described later) as a parameter representing the electron density. 
     The measuring device  70  is not limited to a plasma absorption probe. In another example, the measuring device  70  may be an emission spectroscopic analyzer. In the example, the measuring device  70  specifies the electron density in the chamber  10  from the plasma emission intensity. In yet another example, the measuring device  70  may be a device that specifies the electron density in the chamber  10  using laser light. 
     The plasma processing apparatus  1  further includes a controller MC. The controller MC is a computer that includes a processor, a storage device, an input device, a display device, and the like, and controls the respective units of the plasma processing apparatus  1 . Specifically, the controller MC executes a control program stored in the storage device, and controls the respective units 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 of the controller MC. The etching method according to various embodiments may be executed in the plasma processing apparatus  1  by controlling the respective units of the plasma processing apparatus  1  by the controller MC. 
     Hereinafter, a method MT will be described with reference to  FIGS. 1 and 3 .  FIG. 3  is a timing chart of an example related to the etching method illustrated in  FIG. 1 . In  FIG. 3 , the vertical axis represents radio-frequency power, positive ion density, negative ion density, electron density, and output voltage of the power supply unit  64 . In  FIG. 3 , the fact that the radio-frequency power is ON indicates that the radio-frequency power is supplied for plasma generation, and the fact that the radio-frequency power is OFF indicates that the supply of the radio-frequency power is stopped (afterglow state). In the middle part of the timing chart in  FIG. 3 , the solid line represents the positive ion density, the alternate long and short dash line represents the electron density, and the dotted line represents the negative ion density. 
     Further, reference is also made to  FIGS. 4A to 6B .  FIG. 4A  is a diagram illustrating the state of plasma and a substrate in a period P 1  in the timing chart of  FIG. 3 , and  FIG. 4B  is a diagram illustrating the state of plasma and a substrate in a period P 2  in the timing chart of  FIG. 3 .  FIG. 5A  is a diagram illustrating the state of plasma and a substrate in a period P 31  in the timing chart of  FIG. 3 , and  FIG. 5B  is a diagram illustrating the state of plasma and a substrate in a period P 32  in the timing chart of  FIG. 3 .  FIG. 6A  is a diagram illustrating the state of plasma and a substrate in a period P 4  in the timing chart of  FIG. 3 , and  FIG. 6B  is a diagram for explaining a step ST 5  of the etching method illustrated in  FIG. 1 . In the figures, a circle surrounding “+,” a circle surrounding “−,” a circle surrounding “e,” a circle surrounding “A,” and a circle surrounding “A*” represent a positive ion, a negative ion, an electron, an atom or a molecule, and a radical, respectively. Hereinafter, the method MT will be described by taking, as an example, a case where the plasma processing apparatus  1  is used in the execution. Further, in the following description, control of the respective units of the plasma processing apparatus  1  by the controller MC will also be described. 
     The method MT is executed in a state where the substrate W placed on the support stage  16 . The substrate W is held by the electrostatic chuck  20  on the support stage  16 . In an example, the substrate W includes a base region UR, a film EF, and a mask MK. The film EF is provided on the base region UR. The film EF is a film that is etched in the method MT. The mask MK is provided on the film EF. The mask MK provides an opening on the film EF. In the method MT, the pattern of the mask MK is transferred to the film EF. That is, an opening is formed in the film EF in the method MT. 
     In the method MT, step ST 1  is performed. In the step ST 1 , radio-frequency power is supplied to the lower electrode  18  (or the upper electrode  30 ) in order to generate plasma from the gas in the chamber  10 . The gas may be continuously supplied into the chamber  10  during the execution of the method MT. The radio-frequency power is supplied in the period P 1  and the period P 2 , as illustrated in  FIG. 3 . The period P 1  and the period P 2  are execution periods of the step ST 1 . 
     The controller MC executes the first control in order to execute the step ST 1 . In the first control, the controller MC controls the gas supply to supply gas into the chamber  10 . In the first control, the controller MC controls the exhaust device  50  so as to set the pressure in the chamber  10  to a designated pressure. Further, in the first control, the controller MC controls the radio-frequency power source  61  so as to supply radio-frequency power to the lower electrode  18  (or the upper electrode  30 ). 
     As illustrated in  FIG. 4A , the plasma PL generated in the step ST 1  includes positive ions, negative ions, electrons, atoms or molecules, and radicals. In the plasma PL generated in the step ST 1 , the amount of negative ions is relatively small. 
     Step ST 2  is executed during the execution of the step ST 1 . That is, the step ST 2  is executed during the supply of radio-frequency power for generating plasma. In the step ST 2 , as illustrated in  FIG. 4B , a negative DC voltage is applied to the lower electrode  18  in order to etch the substrate W (i.e., the film EF) with positive ions from the plasma PL generated in the step ST 1 . 
     The controller MC executes the second control in order to execute the step ST 2 . In the second control, the controller MC controls the power supply unit  64  so as to apply a negative DC voltage to the lower electrode  18 . 
     When the step ST 2  is executed, positive ions collide with the substrate W and etch the substrate W. In the step ST 2 , since positive ions are supplied to the substrate W, the substrate W is charged with a positive charge as illustrated in  FIG. 5A . In  FIG. 5A , the symbol “+” in the substrate W indicates that the substrate W is charged with a positive charge. 
     Subsequently, step ST 3  is executed. In the step ST 3 , application of the negative DC voltage to the lower electrode  18  is stopped in order to generate negative ions. Further, in the step ST 3 , the supply of radio-frequency power is stopped. 
     The controller MC executes the third control in order to execute the step ST 2 . In the third control, the controller MC controls the power supply unit  64  so as to apply a negative DC voltage to the lower electrode  18 . Further, in the third control, the controller MC controls the radio-frequency power source  61  to stop supplying radio-frequency power. Also, the gas supply may continuously supply gas to the chamber  10  from the step ST 1 . The exhaust device  50  may continuously adjust the pressure in the chamber  10  from the step ST 1 . 
     In the step ST 3  according to an embodiment, the application of the negative DC voltage to the lower electrode may be stopped before the supply of the radio-frequency power is stopped. In the third control of the embodiment, the controller MC may control the power supply unit  64  so as to stop the application of the negative DC voltage to the lower electrode  18  before stopping the supply of the radio-frequency power to the radio-frequency power source  61 . According to the embodiment, an abnormal discharge is more reliably prevented. 
     In the period immediately after the start of the step ST 3  (i.e., the period P 31  in  FIG. 3 ), the plasma PL includes positive ions, negative ions, electrons, atoms or molecules, and radicals as illustrated in  FIG. 5A . In the plasma PL, the number of negative ions is relatively small. 
     In an embodiment, step STm is performed during execution of the step ST 3 . In the step STm, the above-described parameter representing the electron density in the chamber  10  is measured by the measuring device  70 . The parameter measured by the measuring device  70  is given to the controller MC. 
     In the subsequent step STa, it is determined from the parameter by the controller MC whether the electron density in the chamber  10  is decreased so as to satisfy a predetermined standard. For example, when the electron density becomes smaller than a threshold value, it is determined that the electron density in the chamber  10  is decreased so as to satisfy a predetermined standard. Further, the decrease in the electron density during the step ST 3  reflects an increase in the amount of negative ions in the chamber  10 . 
     When it is determined in the step STa that the electron density in the chamber  10  is not decreased so as to satisfy a predetermined standard, the step ST 3  is continued. That is, the execution of the third control by the controller MC continues. Meanwhile, when it is determined in the step STa that the electron density in the chamber  10  is decreased so as to satisfy a predetermined standard, the step ST 3  ends and the process proceeds to step ST 4 . That is, when it is determined from the parameter that the electron density in the chamber  10  is decreased so as to satisfy a predetermined standard, the controller MC ends the third control and starts executing the fourth control. 
     During the execution of the step ST 3 , electrons are bonded to chemical species such as atoms, molecules, or radicals in the chamber  10  to generate negative ions. At the end of the step ST 3  or a period immediately before the step ST 3  (i.e., the period P 32  in  FIG. 3 ), sufficient negative ions are generated in the chamber  10  as illustrated in  FIG. 5B . 
     In an embodiment, the step STm and the step STa may be omitted. In the embodiment, the step ST 3  (and the third control) may be ended after a predetermined time has elapsed from the start time. The predetermined time is determined in advance as a time necessary for sufficient generation of negative ions in the chamber  10  after the start of the step ST 3 . 
     The step ST 4  is performed in the period P 4  after execution of the step ST 3 . In the step ST 4 , in order to supply the negative ions generated in the step ST 3  to the substrate W, a positive DC voltage is applied to the lower electrode  18  in a state where the supply of radio-frequency power is stopped. 
     The controller MC executes the fourth control in order to execute the step ST 4 . In the fourth control of the embodiment, the controller MC controls the power supply unit  64  to apply a positive DC voltage to the lower electrode  18  in a state where the supply of radio-frequency power to the radio-frequency power source  61  is stopped. Also, the gas supply may continuously supply a gas to the chamber  10  from the step ST 1 . The exhaust device  50  may continuously adjust the pressure in the chamber  10  from the step ST 1 . 
     In the step ST 4 , since a positive DC voltage is applied to the lower electrode  18 , negative ions are attracted to the substrate W, as illustrated in  FIG. 6A . The negative ions collide with the substrate W and etch the substrate W (i.e., the film EF). Negative ions also reduce the amount of positive charges on the substrate W. 
     As described above, in the method MT, a negative DC voltage is supplied to the lower electrode  18  in a state where plasma is generated by supplying radio-frequency power. As a result, positive ions collide with the substrate W and etch the substrate W. Subsequently, the supply of the radio-frequency power and the application of the DC voltage to the lower electrode are stopped. Although the amount of negative ions generated is small in a state where radio-frequency power is supplied, negative ions are efficiently generated by electrons which are attached to chemical species in the gas when the supply of radio-frequency power is stopped. Subsequently, a positive DC voltage is applied to the lower electrode  18  in a state where the supply of radio-frequency power is stopped. As a result, negative ions are supplied to the substrate W. In the method MT, negative ions reduce the amount of positive charges on the substrate. Also, the substrate W is etched using both positive ions and negative ions. Therefore, the etching efficiency is improved. 
     In an embodiment, an etching sequence ESQ including the step ST 1 , the step ST 2 , the step ST 3 , and the step ST 4  is executed one or more times. In the embodiment, the controller MC executes the etching control sequence including the first control, the second control, the third control, and the fourth control one or more times. When the etching sequence ESQ is executed a plurality of times, the repetition frequency of the etching sequence ESQ may be 10 kHz or more and 500 kHz or less. The repetition frequency of the etching sequence ESQ may be 50 kHz or more and 400 kHz or less. Further, the frequency may be greater than 400 kHz. 
     When the etching sequence ESQ is performed a plurality of times, the method MT further includes step STb. In the step STb, it is determined whether the stop condition is satisfied. The stop condition is satisfied when the etching sequence ESQ (or the etching control sequence) is performed a predetermined number of times. When it is determined in the step STb that the stop condition is not satisfied, the etching sequence ESQ (or the etching control sequence) is performed. 
     In an embodiment, when it is determined in the step STb that the stop condition is satisfied, the step ST 5  is performed. In the step ST 5 , the gas is exhausted from the internal space of the chamber  10  by the exhaust device  50 . In the step ST 5 , the supply of radio-frequency power by the radio-frequency power source  61  is stopped, and the application of the DC voltage to the lower electrode  18  by the power supply unit  64  is stopped. 
     The controller MC performs the fifth control in order to execute the step ST 5 . In the fifth control, the controller MC controls the exhaust device  50  so as to discharge a gas from the internal space of the chamber  10 . In the fifth control, the controller MC controls the radio-frequency power source  61  so as to stop the supply of the radio-frequency power. Further, in the fifth control, the controller MC controls the power supply unit  64  so as to stop the application of the DC voltage to the lower electrode  18 . In addition, the gas supply may continuously supply the gas to the chamber  10  from the step ST 1 , or may stop the supply of the gas during the execution period of the step ST 5 . 
     Etching byproducts generated by the etching sequence ESQ may be left in the openings formed in the substrate W. When the step ST 5  is executed, etching byproducts are discharged from the chamber  10  as gas, as illustrated in  FIG. 6B . In  FIG. 6B , a circle surrounding “B” represents an etching byproduct. In  FIG. 5A , the symbol “+” in the substrate W indicates that the substrate W is charged with a positive charge. 
     In an embodiment, another sequence ASQ including one or more executions of the etching sequence ESQ and the step ST 5  may be repeated. The repetition frequency of the sequence ASQ may be 100 Hz or more and 10 kHz or less. A ratio of one or more execution periods of the etching sequence ESQ in one execution period of the sequence ASQ may be 30% or more and 70% or less. In the embodiment, the controller MC repeatedly executes another control sequence. Another control sequence includes one or more executions of the etching control sequence and a fifth control. In the embodiment, as illustrated in  FIG. 1 , the method MT includes step STc. In the step STc, it is determined whether the stop condition is satisfied. In the step STc, it is determined whether the stop condition is satisfied. The stop condition is satisfied when the sequence ASQ (or another control sequence) is executed a predetermined number of times. When it is determined in the step STc that the stop condition is not satisfied, the sequence ASQ (or another control sequence) is executed again. Meanwhile, when it is determined in the step STc that the stop condition is satisfied, the method MT is ended. 
     In an embodiment, during one execution of the sequence ASQ, the step ST 5  may be executed for 10 μsec or more. In the embodiment, the fifth control is executed for 10 μsec or more in the execution period of the other control sequence described above. In the embodiment, etching byproducts are more reliably discharged. As a result, the etching efficiency of the substrate is further improved. 
     In an embodiment, the time length of the execution period of the step ST 5  may be increased as the number of executions of the sequence ASQ increases. In the embodiment, the controller MC may increase the time length of the execution period of the fifth control as the number of executions of the other control sequence increases. In the embodiment, as the depth of the opening formed in the substrate W increases, the time length of the execution period of the step ST 5  is increased. Therefore, etching byproducts are more reliably discharged. 
     In an embodiment, radio-frequency power may be intermittently supplied from the radio-frequency power source  61  to generate plasma during the execution period of the step ST 1 , that is, in the period P 1  and the period P 2 . That is, a plurality of pulses of radio-frequency power may be intermittently supplied from the radio-frequency power supply  61  during the execution period of the step ST 1 . In the embodiment, a plurality of pulses of radio-frequency power may be periodically supplied from the radio-frequency power supply source  61  during the execution period of the step ST 1 . The cycle in which the pulses of the radio-frequency power are supplied from the radio-frequency power source  61  may be a cycle defined by a frequency of 100 kHz or more and 1 MHz or less. In the embodiment, the power levels of the plurality of pulses of the radio-frequency power supplied from the radio-frequency power source  61  during the execution period of the step ST 1  may vary. In the embodiment, an average value of the power levels of the plurality of pulses of the radio-frequency power supplied from the radio-frequency power source  61  during the execution period of the step ST 1  may vary in the repetition of the etching sequence ESQ. 
     In an embodiment, a negative DC voltage may be intermittently applied from the power supply unit  64  to the lower electrode  18  during the execution period of the step ST 2 , that is, in the period P 2 . That is, a plurality of pulses of the negative DC voltage may be intermittently applied from the power supply unit  64  to the lower electrode  18  during the execution period of the step ST 2 . In the embodiment, a plurality of pulses of the negative DC voltage may be periodically applied from the power supply unit  64  to the lower electrode  18  during the execution period of the step ST 2 . The cycle in which the pulses of the negative DC voltage are applied from the power supply unit  64  to the lower electrode  18  may be a cycle defined by a frequency of 100 kHz or more and 1 MHz or less. The timing at which the pulses of the negative DC voltage are applied from the power supply unit  64  to the lower electrode  18  may be synchronized with the timing at which the pulses of the radio-frequency power are supplied from the radio-frequency power source  61 . In the embodiment, the voltage values of a plurality of pulses of the negative DC voltage applied from the power supply unit  64  to the lower electrode  18  during the execution period of the step ST 2  may vary. In the embodiment, an average value of the voltage values of a plurality of pulses of the negative DC voltage applied from the power supply unit  64  to the lower electrode  18  in the execution period of the step ST 2  may vary in the repetition of the etching sequence ESQ. 
     In an embodiment, a positive DC voltage may be intermittently applied from the power supply unit  64  to the lower electrode  18  during the execution period of the step ST 4 , that is, in the period P 4 . That is, a plurality of pulses of the positive DC voltage may be intermittently applied from the power supply unit  64  to the lower electrode  18  during the execution period of the step ST 4 . In the embodiment, a plurality of pulses of the positive DC voltage may be periodically applied from the power supply unit  64  to the lower electrode  18  during the execution period of the step ST 4 . The cycle in which the pulses of the positive DC voltage are applied from the power supply unit  64  to the lower electrode  18  may be a cycle defined by a frequency of 100 kHz or more and 1 MHz or less. In the embodiment, the voltage values of a plurality of pulses of the positive DC voltage applied from the power supply unit  64  to the lower electrode  18  during the execution period of the step ST 4  may vary. In the embodiment, an average value of the voltage values of a plurality of pulses of the positive DC voltage applied from the power supply unit  64  to the lower electrode  18  during the execution period of the step ST 4  may vary in the repetition of the etching sequence ESQ. 
     Although various embodiments have been described above, the present disclosure is not limited to the embodiments described above, and various omissions, substitutions, and changes may be made. In addition, it is possible to combine the elements in other embodiments to form other embodiments. 
     For example, the plasma processing apparatus  1  is a capacitively coupled plasma processing apparatus, but the plasma processing apparatus according to another embodiment may be another type of plasma processing apparatus such as an inductively coupled plasma processing apparatus. Further, the method MT may be performed using any type of plasma processing apparatus other than the plasma processing apparatus  1 , for example, an inductively coupled plasma processing apparatus. 
     According to an embodiment, the amount of positive charges on the substrate may be reduced and the etching efficiency may be increased. 
     From the foregoing, 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.