Patent Publication Number: US-2019198336-A1

Title: Etching method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of Japanese Patent Application No. 2017-251560 filed on Dec. 27, 2017, the entire disclosures of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The various aspects and embodiments described herein pertain generally to an etching method. 
     BACKGROUND 
     In the manufacture of an electronic device, plasma etching is performed to transfer a pattern of a mask to a film of a substrate. In the plasma etching, it is required to etch the film selectively with respect to the mask. That is, selectivity is required in the plasma etching. 
     To achieve high selectivity, there is known an etching method of generating plasma of two kinds of processing gases alternately. One of these two processing gases is a deposition gas, and the other is an etching gas. That is, the one processing gas has higher deposition property than the other. If the plasma of the deposition gas is generated, a deposit is formed on the mask. In the etching of the film with the plasma of the etching gas, the mask is protected by the deposit. This etching method is described in Patent Document 1. 
     In the etching method disclosed in Patent Document 1, plasma etching under a first processing condition and plasma etching under a second processing condition are alternately performed. Both a first processing gas used in the first processing condition and a second processing gas used in the second processing condition include a C 4 F 8  gas and a C 4 F 6  gas. A flow rate of the C 4 F 6  gas in the first processing condition is larger than a flow rate of the C 4 F 6  gas in the second processing condition, and a flow rate of the C 4 F 8  gas in the second processing condition is larger than a flow rate of the C 4 F 8  gas in the first processing condition. 
     Patent Document 1: Japanese Patent Laid-open Publication No. 2012-039048 
     As stated above, in the plasma etching, the film needs to be etched selectively against the mask, that is, selectivity is required. In the plasma etching with the two kinds of fluorocarbon gases as disclosed in Patent Document 1, it is still required to improve the selectivity. 
     SUMMARY 
     In one exemplary embodiment, there is provided an etching method of etching a film of a substrate. The substrate has a mask provided with a pattern on the film. The etching method is performed in a state that the substrate is placed in a chamber of a plasma processing apparatus. The etching method comprises (i) generating plasma of a first processing gas including a first gas containing first fluorocarbon, a second gas containing second fluorocarbon, an oxygen-containing gas and a fluorine-containing gas within the chamber to etch the film; and (ii) generating plasma of a second processing gas including the first gas, the second gas, the oxygen-containing gas and the fluorine-containing gas within the chamber to etch the film. The generating of the plasma of the first processing gas and the generating of the plasma of the second processing gas are performed alternately. A ratio of a number of fluorine atoms to a number of carbon atoms in a molecule of the second fluorocarbon is larger than a ratio of the number of fluorine atoms to the number of carbon atoms in a molecule of the first fluorocarbon. A flow rate of the first gas in the first processing gas is larger than a flow rate of the first gas in the second processing gas. A flow rate of the second gas in the second processing gas is larger than a flow rate of the second gas in the first processing gas. A flow rate of the oxygen-containing gas in the second processing gas is larger than a flow rate of the oxygen-containing gas in the first processing gas. A flow rate of the fluorine-containing gas in the second processing gas is smaller than a flow rate of the fluorine-containing gas in the first processing gas. 
     In the etching method, an overshoot and an undershoot in a time characteristic of an emission intensity of fluorine in the plasma and a time characteristic of an emission intensity of oxygen in the plasma are suppressed. Further, each of the emission intensity of the fluorine in the plasma and the emission intensity of the oxygen in the plasma increases or decreases as time passes by. That is, it is possible to increase or decrease density of plasma of the fluorine and density of plasma of the oxygen with a lapse of time while suppressing an excessive variation in the density of the plasma of the fluorine and the density of the plasma of the oxygen. Thus, the amount of the carbon-containing material deposited on the mask can be controlled. Therefore, it becomes possible to etch the film selectively with respect to the mask, that is, to achieve the high selectivity. 
     A high frequency power for generation of the plasma of the first processing gas and generation of the plasma of the second processing gas is continuously supplied through the generating of the plasma of the first processing gas and the generating of the plasma of the second processing gas. 
     The flow rate of the first gas in the first processing gas is larger than the flow rate of the second gas in the first processing gas, and the flow rate of the second gas in the second processing gas is larger than the flow rate of the first gas in the second processing gas. 
     The first fluorocarbon is perfluorocarbon or hydrofluorocarbon, and the second fluorocarbon is perfluorocarbon or hydrofluorocarbon. The first fluorocarbon may be C 4 F 6  and the second fluorocarbon may be C 4 F 8 . The oxygen-containing gas may be an oxygen gas (O 2  gas). The fluorine-containing gas may be a NF 3  gas. 
     According to the exemplary embodiment as stated above, it is possible to etch the film more selectively with respect to the mask, that is, to achieve the high selectivity. 
     The foregoing summary is illustrative only and is not intended to be 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 
       In the detailed description that follows, embodiments are described as illustrations only since various changes and modifications will become apparent to those skilled in the art from the following detailed description. The use of the same reference numbers in different figures indicates similar or identical items. 
         FIG. 1  is a flowchart illustrating an etching method according to an exemplary embodiment; 
         FIG. 2  is a partially enlarged cross sectional view illustrating an example of a substrate to which the etching method shown in  FIG. 1  is applicable; 
         FIG. 3  is a diagram schematically illustrating an example of a plasma processing apparatus which can be used to perform the etching method shown in  FIG. 1 ; 
         FIG. 4  is a timing chart regarding the etching method shown in  FIG. 1 ; 
         FIG. 5A  is a graph showing a time characteristic of an emission intensity of a wavelength of 704 nm measured in a first experiment;  FIG. 5B , a graph showing a time characteristic of an emission intensity of a wavelength of 777 nm measured in the first experiment; and  FIG. 5C , a graph showing a time characteristic of an emission intensity of a wavelength of 516 nm measured in the first experiment; 
         FIG. 6A  is a graph showing a time characteristic of an emission intensity of a wavelength of 704 nm measured in a second experiment;  FIG. 6B , a graph showing a time characteristic of an emission intensity of a wavelength of 777 nm measured in the second experiment; and  FIG. 6C , a graph showing a time characteristic of an emission intensity of a wavelength of 516 nm measured in the second experiment; 
         FIG. 7A  is a graph showing a time characteristic of an emission intensity of a wavelength of 704 nm measured in a first comparative experiment;  FIG. 7B , a graph showing a time characteristic of an emission intensity of a wavelength of 777 nm measured in the first comparative experiment; and  FIG. 7C , a graph showing a time characteristic of an emission intensity of a wavelength of 516 nm measured in the first comparative experiment; and 
         FIG. 8A  is a graph showing a time characteristic of an emission intensity of a wavelength of 704 nm measured in a second comparative experiment;  FIG. 8B , a graph showing a time characteristic of an emission intensity of a wavelength of 777 nm measured in the second comparative experiment; and  FIG. 8C , a graph showing a time characteristic of an emission intensity of a wavelength of 516 nm measured in the second comparative experiment. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to the accompanying drawings, which form a part of the description. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. Furthermore, unless otherwise noted, the description of each successive drawing may reference features from one or more of the previous drawings to provide clearer context and a more substantive explanation of the current exemplary embodiment. Still, the exemplary embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented herein. It will be readily understood that the aspects of the present disclosure, as generally described herein and illustrated in the drawings, may be arranged, substituted, combined, separated, and designed in a wide variety of different configurations, all of which are explicitly contemplated herein. 
     Hereinafter, various exemplary embodiments will be described in detail with reference to the accompanying drawings. In the various drawings, same or corresponding parts will be assigned same reference numerals. 
       FIG. 1  is a flowchart for describing an etching method according to an exemplary embodiment. The etching method (hereinafter, referred to as “method MT”) shown in  FIG. 1  is performed to etch a film of a substrate.  FIG. 2  is a partially enlarged cross sectional view of an example of a substrate to which the etching method shown in  FIG. 1  is applicable. A substrate W shown in  FIG. 2  has a film EF and a mask MK. The film EF is an etching target film and is provided on an underlying region UR. The film EF is a silicon-containing film. The film EF may be, but not limited to, a silicon oxide film, a silicon nitride film or a multilayered film including a plurality of silicon oxide films and a multiplicity of silicon nitride films. In the multilayered film, the plurality of silicon oxide films and the multiplicity of silicon nitride films are alternately stacked on top of each other. The mask MK is provided on the film EF. The mask MK is made of a carbon-containing material or polycrystalline silicon. The mask MK is provided with a pattern to be transferred to the film EF. A surface of the film EF is partially exposed through the pattern of the mask MK. The mask MK provides one or more openings such as a hole and/or a groove. 
     A plasma processing apparatus is used to perform the method MT.  FIG. 3  is a diagram schematically illustrating an example of a plasma processing apparatus which can be used to perform the etching method shown in  FIG. 1 . A plasma processing apparatus  1  shown in  FIG. 3  is configured as a capacitively coupled plasma etching apparatus. The plasma processing apparatus  1  is equipped with a chamber  10 . The chamber  10  has an internal space  10   s  therein. 
     The chamber  10  includes a chamber main body  12 . The chamber main body  12  has a substantially cylindrical shape. The internal space  10   s  is provided inside the chamber main body  12 . The chamber main body  12  is made of, by way of non-limiting example, aluminum. A film having corrosion resistance is formed on an inner wall surface of the chamber main body  12 . The film having the corrosion resistance may be a film made of ceramic such as aluminum oxide or yttrium oxide. 
     A passage  12   p  is formed at a sidewall of the chamber main body  12 . When the substrate W is transferred between the internal space  10   s  and an outside of the chamber  10 , the substrate W passes through the passage  12   p . This passage  12   p  is opened or closed by a gate valve  12   g . The gate valve  12   g  is provided along the sidewall of the chamber main body  12 . 
     A supporting member  13  is provided on a bottom portion of the chamber main body  12 . The supporting member  13  is made of an insulating material and has a substantially cylindrical shape. The supporting member  13  is extended upwards from the bottom portion of the chamber main body  12  within the internal space  10   s . The supporting member  13  is configured to support a supporting table  14 . The supporting table  14  is provided within the internal space  10   s . The supporting table  14  is configured to support the substrate W within the internal space  10   s.    
     The supporting table  14  includes a lower electrode  18  and an electrostatic chuck  20 . The supporting table  14  may further include an electrode plate  16 . The electrode plate  16  is made of a conductor such as, but not limited to, aluminum and has a substantially disk shape. The lower electrode  18  is provided on the electrode plate  16 . The lower electrode  18  is made of a conductor such as, but not limited to, aluminum and has a substantially disk shape. The lower electrode  18  is electrically connected to the electrode plate  16 . 
     The electrostatic chuck  20  is provided on the lower electrode  18 . The substrate W is placed on a top surface of the electrostatic chuck  20 . The electrostatic chuck  20  has a main body and an electrode. The main body of the electrostatic chuck  20  is made of a dielectric material. The electrode of the electrostatic chuck  20  is a film-shaped electrode and is provided within the main body of the electrostatic chuck  20 . The electrode of the electrostatic chuck  20  is connected to a DC power supply  20   p  via a switch  20   s . If a voltage from the DC power supply  20   p  is applied to the electrode of the electrostatic chuck  20 , an electrostatic attracting force is generated between the electrostatic chuck  20  and the substrate W. Thus, the substrate W is attracted to and held by the electrostatic chuck  20  by the generated electrostatic attracting force. 
     A focus ring FR is provided on a peripheral portion of the lower electrode  18  to surround an edge of the substrate W. The focus ring FR is configured to improve uniformity of a plasma processing upon the substrate W within a surface thereof. The focus ring FR may be made of, but not limited to, silicon, silicon carbide or quartz. 
     A path  18   f  is provided within the lower electrode  18 . A heat exchange medium (for example, a coolant) is supplied via a pipeline  22   a  into the path  18   f  from a chiller unit  22  provided at the outside of the chamber  10 . The heat exchange medium supplied into the path  181  is returned back into the chiller unit  22  via a pipeline  22   b . In the plasma processing apparatus  1 , a temperature of the substrate W placed on the electrostatic chuck  20  is adjusted by a heat exchange between the heat exchange medium and the lower electrode  18 . 
     The plasma processing apparatus  1  is equipped with a gas supply line  24 . Through the gas supply line  24 , a heat transfer gas (e.g., a He gas) from a heat transfer gas supply mechanism is supplied into a gap between the top surface of the electrostatic chuck  20  and a rear surface of the substrate W. 
     The plasma processing apparatus  1  is further equipped with an upper electrode  30 . The upper electrode  30  is provided above the supporting table  14 . The upper electrode  30  is supported at an upper portion of the chamber main body  12  with a member  32  therebetween. The member  32  is made of a material having insulation property. The upper electrode  30  and the member  32  close a top opening of the chamber main body  12 . 
     The upper electrode  30  may include a ceiling plate  34  and a supporting body  36 . A bottom surface of the ceiling plate  34  is a surface facing the internal space  10   s , and forms and confines the internal space  10   s . The ceiling plate  34  may be made of a conductor or a semiconductor having low Joule heat. The ceiling plate  34  is provided with multiple gas discharge holes  34   a . These gas discharge holes  34   a  are formed through the ceiling plate  34  in a plate thickness direction. 
     The supporting body  36  is configured to support the ceiling plate  34  in a detachable manner, and is made of a conductive material such as, but not limited to, aluminum. A gas diffusion space  36   a  is provided within the supporting body  36 . The supporting body  36  is provided with multiple gas holes  36   b . The multiple gas holes  36   b  are extended downwards from the gas diffusion space  36   a . The multiple gas holes  36   b  communicate with the multiple gas discharge holes  34   a  respectively. The supporting body  36  is provided with a gas inlet port  36   c . The gas inlet port  36   c  is connected to the gas diffusion space  36   a . A gas supply line  38  is connected to this gas inlet port  36   c.    
     The gas supply line  38  is connected to a gas source group  40  via a valve group  41 , a flow rate controller group  42  and a valve group  43 . The gas source group  40  includes a plurality of gas sources. The plurality of gas sources belonging to the gas source group  40  include sources of gases for use in the method MT. Each of the valve groups  41  and  43  includes a plurality of opening/closing valves. The flow rate controller group  42  includes a plurality of flow rate controllers. Each of the plurality of flow rate controllers belonging to the flow rate controller group  42  may be a mass flow controller or a pressure control type flow rate controller. Each of the plurality of gas sources belonging to the gas source group  40  is connected to the gas supply line  38  via a corresponding opening/closing valve belonging to the valve group  41 , a corresponding flow rate controller belonging to the flow rate controller group  42  and a corresponding opening/closing valve belonging to the valve group  43 . 
     In the plasma processing apparatus  1 , a shield  46  is provided along the inner wall surface of the chamber main body  12  in a detachable manner. The shield  46  is also provided on an outer side surface of the supporting member  13 . The shield  46  is configured to suppress an etching byproduct from adhering to the chamber main body  12 . The shield  46  is prepared by forming a film having corrosion resistance on a surface of a base member made of, by way of non-limiting example, aluminum. The film having corrosion resistance may be one made of ceramic such as yttrium oxide. 
     A baffle plate  48  is provided between the supporting member  13  and the sidewall of the chamber main body  12 . The baffle plate  48  may be made of, by way of example, an aluminum base member on which a film having corrosion resistance is formed. The film having corrosion resistance may be one made of ceramic such as yttrium oxide. The baffle plate  48  is provided with a plurality of through holes. A gas exhaust port  12   e  is provided at the bottom portion of the chamber main body  12  under the baffle plate  48 . The gas exhaust port  12   e  is connected with a gas exhaust device  50  via a gas exhaust line  52 . The gas exhaust device  50  has a pressure control valve and a vacuum pump such as a turbo molecular pump. 
     The plasma processing apparatus  1  is further equipped with a first high frequency power supply  62  and a second high frequency power supply  64 . The first high frequency power supply  62  is configured to generate a first high frequency power for plasma generation. A frequency of the first high frequency power is in a range from, e.g., 27 MHz to 100 MHz. The first high frequency power supply  62  is connected to the lower electrode  18  via a matching device  66  and the electrode plate  16 . The matching device  66  is equipped with a circuit configured to match an output impedance of the first high frequency power supply  62  and an input impedance at a load side (lower electrode  18  side). Further, the first high frequency power supply  62  may be connected to the upper electrode  30  via the matching device  66 . 
     The second high frequency power supply  64  is configured to generate a second high frequency power for ion attraction into the substrate W. A frequency of the second high frequency power is lower than the frequency of the first high frequency power. The frequency of the second high frequency power falls within a range from, e.g., 400 kHz to 13.56 MHz. The second high frequency power supply  64  is connected to the lower electrode  18  via a matching device  68  and the electrode plate  16 . The matching device  68  is equipped with a circuit configured to match an output impedance of the second high frequency power supply  64  and the input impedance at the load side (lower electrode  18  side). 
     The plasma processing apparatus  1  may further include a DC power supply  70 . The DC power supply  70  is connected to the upper electrode  30 . The DC power supply  70  is configured to generate a negative DC voltage and apply the generated DC voltage to the upper electrode  30 . 
     The plasma processing apparatus  1  may further include a control unit  80 . The control unit  80  may be implemented by a computer including a processor, a storage unit such as a memory, an input device, a display device, a signal input/output interface, and so forth. The control unit  80  is configured to control individual components of the plasma processing apparatus  1 . In the control unit  80 , an operator can input commands through the input device to manage the plasma processing apparatus  1 . Further, in the control unit  80 , an operational status of the plasma processing apparatus  1  can be visually displayed on the display device. Further, the storage unit of the control unit  80  stores therein control programs and recipe data. The control programs are executed by the processor of the control unit  80  to perform various processings in the plasma processing apparatus  1 . As the processor of the control unit  80  executes the control programs and controls the individual components of the plasma processing apparatus  1  according to the recipe data, the method MT is performed in the plasma processing apparatus  1 . 
     Now, the method MT will be described for an example case where the method MT is performed on the substrate W shown in  FIG. 2  by using the plasma processing apparatus  1 . The substrate to which the method MT is applied may not be particularly limited as long as the substrate has a film and a mask having a pattern to be transferred to the film. In the following description, reference is made of  FIG. 4  as well as  FIG. 1 .  FIG. 4  is a timing chart regarding the etching method shown in  FIG. 1 . 
     The method MT is performed in a state that the substrate W is placed within the chamber of the plasma processing apparatus  1 , that is, within the internal space  10   s . Within the internal space  10   s , the substrate W is placed on and held by the electrostatic chuck  20 . As shown in  FIG. 1  and  FIG. 4 , the method MT includes a process ST 1  and a process ST 2 . The process ST 1  and the process ST 2  are alternately performed. 
     In the process ST 1 , plasma of a first processing gas is generated within the chamber  10 , that is, within the internal space  10   s  to etch the film EF. In the process ST 2 , plasma of a second processing gas is generated within the chamber  10 , that is, within the internal space  10   s  to etch the film EF. Each of the first processing gas and the second processing gas includes a first gas, a second gas, an oxygen-containing gas and a fluorine-containing gas. 
     The first gas includes first fluorocarbon. The first fluorocarbon may be perfluorocarbon or hydrofluorocarbon. The second gas includes second fluorocarbon. The second fluorocarbon may be perfluorocarbon or hydrofluorocarbon. A value of a ratio of the number of fluorine atoms to the number of carbon atoms in a molecule of the second fluorocarbon is larger than a value of a ratio of the number of fluorine atoms to the number of carbon atoms in a molecule of the first fluorocarbon. As an example, the first fluorocarbon is C 4 F 6 , and the second fluorocarbon is C 4 F 8 . As another example, the first fluorocarbon is C 4 F 6 , and the second fluorocarbon is CHF 3 . The oxygen-containing gas included in each of the first processing gas and the second processing gas may be an oxygen gas (O 2  gas), carbon monoxide gas or carbon dioxide gas. The fluorine-containing gas included in each of the first processing gas and the second processing gas may not be particularly limited and may be, by way of non-limiting example, a NF 3  gas or a SF 6  gas. As an example, each of the first processing gas and the second processing gas includes the first gas containing C 4 F 6 , the second gas containing C 4 F 8 , the oxygen gas ( 0   2  gas) and the NF 3  gas. 
     As shown in  FIG. 4 , a flow rate of the first gas in the first processing gas is larger than a flow rate of the first gas in the second processing gas. That is, the flow rate of the first gas in the process ST 1  is larger than the flow rate of the first gas in the process ST 2 . Further, a flow rate of the second gas in the second processing gas is larger than a flow rate of the second gas in the first processing gas. That is, the flow rate of the second gas in the process ST 2  is larger than the flow rate of the second gas in the process ST 1 . Further, a flow rate of the oxygen-containing gas in the second processing gas is larger than a flow rate of the oxygen-containing gas in the first processing gas. That is, the flow rate of the oxygen-containing gas in the process ST 2  is larger than the flow rate of the oxygen-containing gas in the process ST 1 . In addition, a flow rate of the fluorine-containing gas in the second processing gas is smaller than a flow rate of the fluorine-containing gas in the first processing gas. That is, the flow rate of the fluorine-containing gas in the process ST 2  is smaller than the flow rate of the fluorine-containing gas in the process ST 1 . Further, the flow rate of the first gas in the first processing gas is larger than the flow rate of the second gas in the first processing gas, and the flow rate of the second gas in the second processing gas is larger than the flow rate of the first gas in the second processing gas. 
     In the process ST 1 , the first processing gas is supplied into the internal space  10   s  from the gas source group  40 . In the process ST 1 , the gas exhaust device  50  is controlled such that a pressure within the internal space  10   s  is set to be a preset pressure. In the process ST 1 , the first high frequency power is supplied to generate plasma of the first processing gas. In the process ST 1 , the second high frequency power may be supplied to the lower electrode  18 . 
     In the process ST 2 , the second processing gas is supplied into the internal space  10   s  from the gas source group  40 . In the process ST 2 , the gas exhaust device  50  is controlled such that the pressure within the internal space  10   s  is set to be a predetermined pressure. In the process ST 2 , the first high frequency power is supplied to generate plasma of the second processing gas. Further, in the process ST 2 , the second high frequency power is supplied to the lower electrode  18 . In the exemplary embodiment, the first high frequency power is continuously supplied through the process ST 1  and the process ST 2 , that is, through the alternate repetitions of the process ST 1  and the process ST 2 . The second high frequency power may be continuously supplied through the process ST 1  and the process ST 2 , that is, through the alternate repetitions of the process ST 1  and the process ST 2 . 
     The flow rate of the first gas in the first processing gas is large, as compared to that in the second processing gas. The first gas contains a relatively large amount of carbon atoms. Accordingly, while the process ST 1  is being performed, a deposit including a carbon-containing material, that is, a deposit containing carbon and/or carbon and fluorine is formed on the mask MK. The flow rate of the second gas in the second processing gas is large, as compared to that in the first processing gas. The second gas contains a relatively large amount of fluorine atoms. Accordingly, while the process ST 2  is being performed, the film EF is etched. Further, during the process ST 2 , the mask MK is protected by the deposit formed in the process ST 1 . 
     In the method MT, an overshoot and an undershoot in a time characteristic of an emission intensity of fluorine in the plasma and a time characteristic of an emission intensity of oxygen in the plasma are suppressed. Further, each of the emission intensity of the fluorine in the plasma and the emission intensity of the oxygen in the plasma increases or decreases as time passes by. That is, it is possible to increase or decrease density of plasma of the fluorine and density of plasma of the oxygen with a lapse of time while suppressing an excessive variation in the density of the plasma of the fluorine and the density of the plasma of the oxygen. Thus, according to the method MT, the amount of the carbon-containing material deposited on the mask MK can be controlled. Therefore, it becomes possible to etch the film EF selectively with respect to the mask MK, that is, to achieve the high selectivity. 
     So far, various exemplary embodiments have been described. However, the above-described exemplary embodiments are not limiting, and various changes and modifications may be made. By way of example, the method MT may be performed by using any of various types of plasma processing apparatuses such as an inductively coupled plasma processing apparatus and a plasma processing apparatus configured to excite a gas by using a surface wave such as a microwave. Further, in the method MT, either the process ST 1  or the process ST 2  may be first performed. 
     Now, various experiments conducted to evaluate the method MT will be explained. Here, however, it should be noted that the present disclosure is not limited to the following experiments. 
     First and Second Experiments, and First and Second Comparative Experiments 
     In a first experiment and a second experiment, the method MT is performed by using the plasma processing apparatus  1  under the following conditions. Then, the time characteristics (time variations) of the emission intensity of a wavelength of 704 nm (emission intensity of fluorine F), the emission intensity of a wavelength of 777 nm (emission intensity of oxygen O) and the emission intensity of a wavelength of 516 nm (emission intensity of C 2 ) in the internal space  10   s  are measured. 
     Conditions for First Experiment 
     Process ST 1   
     
         
         
           
             C 4 F 6  gas: 87 sccm 
             C 4 F 8  gas: 17 sccm 
             O 2  gas: 47 sccm 
             NF 3  gas: 35 sccm 
             Pressure within internal space  10   s:  1.33 Pa (10 mTorr) 
             First high frequency power: 40 MHz, 1500 W 
             Second high frequency power: 400 kHz, 14000 W 
             Processing time: 60 sec 
           
         
       
    
     Process ST 2   
     
         
         
           
             C 4 F 6  gas: 17 sccm 
             C 4 F 8  gas: 87 sccm 
             O 2  gas: 87 sccm 
             NF 3  gas: 5 sccm 
             Pressure within internal space  10   s:  1.33 Pa (10 mTorr) 
             First high frequency power: 40 MHz, 1500 W 
             Second high frequency power: 400 kHz, 14000 W 
             Processing time: 60 sec 
           
         
       
    
     Conditions for Second Experiment 
     Process ST 1   
     
         
         
           
             C 4 F 6  gas: 87 sccm 
             CHF 3  gas: 34 sccm 
             O 2  gas: 47 sccm 
             NF 3  gas: 35 sccm 
             Pressure within internal space  10   s:  1.33 Pa (10 mTorr) 
             First high frequency power: 40 MHz, 1500 W 
             Second high frequency power: 400 kHz, 14000 W 
             Processing time: 60 sec 
           
         
       
    
     Process ST 2   
     
         
         
           
             C 4 F 6  gas: 17 sccm 
             CHF 3  gas: 174 sccm 
             O 2  gas: 87 sccm 
             NF 3  gas: 5 sccm 
             Pressure within internal space  10   s:  1.33 Pa (10 mTorr) 
             First high frequency power: 40 MHz, 1500 W 
             Second high frequency power: 400 kHz, 14000 W 
             Processing time: 60 sec 
           
         
       
    
     In a first comparative experiment and a second comparative experiment, a first process and a second process specified as follows are alternately repeated by using the plasma processing apparatus  1 . Then, the time characteristics (time variations) of the emission intensity of the wavelength of 704 nm (emission intensity of fluorine F), the emission intensity of the wavelength of 777 nm (emission intensity of oxygen O) and the emission intensity of the wavelength of 516 nm (emission intensity of C 2 ) in the internal space  10   s  are measured. 
     Conditions for First Comparative Experiment 
     First Process 
     
         
         
           
             C 4 F 6  gas: 87 sccm 
             C 4 F 8  gas: 17 sccm 
             O 2  gas: 47 sccm 
             NF 3  gas: 35 sccm 
             Pressure within internal space  10   s:  1.33 Pa (10 mTorr) 
             First high frequency power: 40 MHz, 1500 W 
             Second high frequency power: 400 kHz, 14000 W 
             Processing time: 60 sec 
           
         
       
    
     Second Process 
     
         
         
           
             C 4 F 6  gas: 17 sccm 
             C 4 F 8  gas: 87 sccm 
             O 2  gas: 47 sccm 
             NF 3  gas: 35 sccm 
             Pressure within internal space  10   s:  1.33 Pa (10 mTorr) 
             First high frequency power: 40 MHz, 1500 W 
             Second high frequency power: 400 kHz, 14000 W 
             Processing time: 60 sec 
           
         
       
    
     Conditions for Second Comparative Experiment 
     First Process 
     
         
         
           
             C 4 F 6  gas: 87 sccm 
             C 4 F 8  gas: 17 sccm 
             O 2  gas: 47 sccm 
             NF 3  gas: 35 sccm 
             Pressure within internal space  10   s:  1.33 Pa (10 mTorr) 
             First high frequency power: 40 MHz, 1500 W 
             Second high frequency power: 400 kHz, 14000 W 
             Processing time: 60 sec 
           
         
       
    
     Second Process 
     
         
         
           
             C 4 F 6  gas: 17 sccm 
             C 4 F 8  gas: 87 sccm 
             O 2  gas: 87 sccm 
             NF 3  gas: 35 sccm 
             Pressure within internal space  10   s:  1.33 Pa (10 mTorr) 
             First high frequency power: 40 MHz, 1500 W 
             Second high frequency power: 400 kHz, 14000 W 
             Processing time: 60 sec 
           
         
       
    
       FIG. 5A  is a graph showing the time characteristic of the emission intensity of the wavelength of 704 nm measured in the first experiment;  FIG. 5B , a graph showing the time characteristic of the emission intensity of the wavelength of 777 nm measured in the first experiment; and  FIG. 5C , a graph showing the time characteristic of the emission intensity of the wavelength of 516 nm measured in the first experiment.  FIG. 6A  is a graph showing the time characteristic of the emission intensity of the wavelength of 704 nm measured in the second experiment;  FIG. 6B , a graph showing the time characteristic of the emission intensity of the wavelength of 777 nm measured in the second experiment; and  FIG. 6C , a graph showing the time characteristic of the emission intensity of the wavelength of 516 nm measured in the second experiment.  FIG. 7A  is a graph showing the time characteristic of the emission intensity of the wavelength of 704 nm measured in the first comparative experiment;  FIG. 7B , a graph showing the time characteristic of the emission intensity of the wavelength of 777 nm measured in the first comparative experiment; and  FIG. 7C , a graph showing the time characteristic of the emission intensity of the wavelength of 516 nm measured in the first comparative experiment.  FIG. 8A  is a graph showing the time characteristic of the emission intensity of the wavelength of 704 nm measured in the second comparative experiment;  FIG. 8B , a graph showing the time characteristic of the emission intensity of the wavelength of 777 nm measured in the second comparative experiment; and  FIG. 8C , a graph showing the time characteristic of the emission intensity of the wavelength of 516 nm measured in the second comparative experiment. 
     In the first comparative experiment, the flow rate of the O 2  gas and the flow rate of the NF 3  gas are not changed throughout the first process and the second process. In the first comparative experiment, an overshoot and an undershoot are found in the time characteristic of the emission intensity of the fluorine and the time characteristic of the emission intensity of the oxygen, as shown in  FIG. 7A  and  FIG. 7B . In the second comparative experiment, though the flow rate of the O 2  gas in the second process is increased with respect to the flow rate of the O 2  gas in the first process, the flow rate of the NF 3  gas is not changed throughout the first process and the second process. In this second comparative experiment, an overshoot and an undershoot are found in the time characteristic of the emission intensity of the fluorine, as depicted in  FIG. 8A . In the first comparative experiment and the second comparative experiment, the processing time of the first process and the processing time of the second process are respectively set to be 60 seconds which is relatively long. If, however, the processing time of the first process and the processing time of the second process are short, a state in which the emission intensity of the fluorine and the emission intensity of the oxygen are relatively high is maintained in each of the first process and the second process due to an influence of the overshoot. That is, in case that the processing time of the first process and the processing time of the second process are short, the state in which the density of the plasma of the fluorine and the density of the plasma of the oxygen are relatively high is maintained in each of the first process and the second process. Accordingly, if the flow rate of the O 2  gas and the flow rate of the NF 3  gas are not changed throughout the first process and the second process and if the flow rate of the NF 3  gas is not changed throughout the first process and the second process, the mask is etched, so that the selectivity is deteriorated. 
     Meanwhile, in the first experiment and the second experiment, the time characteristic of the emission intensity of the fluorine and the time characteristic of the emission intensity of the oxygen exhibit neither the overshoot nor the undershoot, as can be seen from  FIG. 5A ,  FIG. 5B ,  FIG. 6A  and  FIG. 6B . Further, the emission intensity of the fluorine is found to increase or decrease apparently in the time characteristic of the emission intensity of the fluorine, and the emission intensity of the oxygen is found to increase or decrease apparently in the time characteristic of the emission intensity of the oxygen. Thus, it is found out that, according to the method MT, the density of the plasma of the fluorine and the density of the plasma of the oxygen can be increased or decreased while suppressing the excessive variations in the density of the plasma of the fluorine and the density of the plasma of the oxygen. 
     Third Experiment and Third Comparative Experiment 
     In a third experiment, a film of a sample substrate is etched by performing the method MT in the plasma processing apparatus  1  under the following conditions. The sample substrate has the etching target film; and a mask provided on the etching target film. The etching target film of the sample substrate is a silicon oxide film. The mask of the sample substrate is made of polycrystalline silicon. In the third experiment, a ratio of a decrement of a film thickness of the etching target film of the sample substrate by the etching to a decrement of a film thickness of the mask of the sample substrate by the etching, that is, a selectivity is calculated. 
     Conditions for Third Experiment 
     Process ST 1   
     
         
         
           
             C 4 F 6  gas: 97 sccm 
             C 4 F 8  gas: 7 sccm 
             O 2  gas: 27 sccm 
             NF 3  gas: 35 sccm 
             Pressure within internal space  10   s:  1.33 Pa (10 mTorr) 
             First high frequency power: 40 MHz, 1500 W 
             Second high frequency power: 400 kHz, 14000 W 
             Processing time: 5 sec 
           
         
       
    
     Process ST 2   
     
         
         
           
             C 4 F 6  gas: 27 sccm 
             C 4 F 8  gas: 77 sccm 
             O 2  gas: 67 sccm 
             NF 3  gas: 5 sccm 
             Pressure within internal space  10   s:  1.33 Pa (10 mTorr) 
             First high frequency power: 40 MHz, 1500 W 
             Second high frequency power: 400 kHz, 14000 W 
             Processing time: 5 sec 
             Number of alternate repetitions of process ST 1  and process ST 2 : 9 times 
           
         
       
    
     In a third comparative experiment, an etching target film of a sample substrate, which is the same as the sample substrate of the third experiment, is etched by performing a first process and a second process as follows alternately in the plasma processing apparatus  1 . In the third comparative experiment, a ratio of a decrement of a film thickness of the etching target film of the sample substrate by the etching to a decrement of a film thickness of the mask of the sample substrate by the etching, that is, a selectivity is calculated. 
     Conditions for Third Comparative Experiment 
     First Process 
     
         
         
           
             C 4 F 6  gas: 77 sccm 
             C 4 F 8  gas: 27 sccm 
             O 2  gas: 47 sccm 
             NF 3  gas: 5 sccm 
             Pressure within internal space  10   s:  1 . 33   Pa (10 mTorr) 
             First high frequency power: 40 MHz, 1500 W 
             Second high frequency power: 400 kHz, 14000 W 
             Processing time: 5 sec 
           
         
       
    
     Second Process 
     C 4 F 6  gas: 27 sccm
         C 4 F 8  gas: 77 sccm   O 2  gas: 47 sccm   NF 3  gas: 5 sccm   Pressure within internal space  10   s:  1.33 Pa (10 mTorr)   First high frequency power: 40 MHz, 1500 W   Second high frequency power: 400 kHz, 14000 W   Processing time: 5 sec   Number of alternate repetitions of first process and second process: 9 times       

     In the third experiment, the selectivity is found to be 4.03. Meanwhile, in the third comparative experiment, the selectivity is found to be 3.18. That is, the selectivity in the third experiment is found to be improved by about 27% as compared to the selectivity in the third comparative experiment. Thus, it is found out that the selectivity can be improved according to the method MT. 
     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. The scope of the inventive concept is defined by the following claims and their equivalents rather than by the detailed description of the exemplary embodiments. It shall be understood that all modifications and embodiments conceived from the meaning and scope of the claims and their equivalents are included in the scope of the inventive concept.