Patent Publication Number: US-2023135998-A1

Title: Plasma processing method and plasma processing system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority under 35 U.S.C. § 119 to Japanese Patent Application No. 2021-178604 filed on Nov. 1, 2021, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     1. Field 
     Exemplary embodiments of the present disclosure relate to a plasma processing method and a plasma processing system. 
     2. Related Art 
     JP 2016-039310 A discloses a method for etching multilayer films having dielectric constants that are different from each other. 
     SUMMARY 
     A plasma processing method performed in a plasma processing apparatus having a chamber is provided in an exemplary embodiment of the present disclosure. This method comprises: (a) providing a substrate having a film stack including a silicon oxide film and a silicon nitride film onto a substrate support in the chamber; and (b) forming a plasma from a processing gas containing HF gas and at least one of C x F y  gas (where x and y are integers equal to or greater than 1) and phosphorus-containing gas to etch the film stack, wherein, in (b), the substrate support is controlled to a temperature of 0° C. or more and 70° C. or less, and a bias RF signal of 10 kW or more or a bias DC signal of 4 kV or more is supplied to the substrate support. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a figure schematically illustrating an exemplary plasma processing system. 
         FIG.  2    is a flowchart showing an example of the processing method. 
         FIG.  3    is a figure schematically illustrating an example of a cross-sectional structure of a substrate W. 
         FIG.  4    is a graph showing the measurement results from Experiment 1. 
         FIG.  5    is a graph showing the measurement results from Experiment 2. 
         FIG.  6    is a graph showing the measurement results from Experiment 3. 
         FIG.  7    is a graph showing the measurement results from Experiment 4. 
         FIG.  8    is a flowchart showing a modified example of the processing method. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure will now be described. 
     One exemplary embodiment is a plasma processing method performed in a plasma processing apparatus having a chamber, the method comprising: (a) providing a substrate having film stack including a silicon oxide film and a silicon nitride film onto a substrate support in the chamber; and (b) forming a plasma from a processing gas containing HF gas and at least one of C x F y  gas (where x and y are integers equal to or greater than 1) and phosphorus-containing gas to etch the film stack, wherein, in (b), the substrate support is controlled to a temperature of 0° C. or more and 70° C. or less, and a bias RF signal of 10 kW or more or a bias DC signal of 4 kV or more is supplied to the substrate support. 
     In one exemplary embodiment, the substrate has a carbon-containing film defining at least one opening on the film stack. 
     In one exemplary embodiment, the carbon-containing film is a photoresist film, a spin-on carbon film, or an amorphous carbon film. 
     In one exemplary embodiment, the C x F y  gas includes at least one gas selected from the group consisting of C 3 F 6  gas, C 4 F 8  gas, C 4 F 6  gas, and C 3 F 8  gas. 
     In one exemplary embodiment, the phosphorus-containing gas is a phosphorus halide gas. 
     In one exemplary embodiment, the phosphorus halide gas is phosphorus fluoride. 
     In one exemplary embodiment, the phosphorus-containing gas is at least one gas selected from the group consisting of PF 3  gas, PF 5  gas, POF 3  gas, HPF 6  gas, PCl 3  gas, PCl 5  gas, POCl 3  gas, PBr 3  gas, PBr 5  gas, POBr 3  gas, PI 3  gas, P 4 O 10  gas, P 4 O 8  gas, P 4 O 6  gas, PH 3  gas, Ca 3 P 2  gas, H 3 PO 4  gas, and Na 3 PO 4  gas. 
     In one exemplary embodiment, the processing gas further includes an oxygen-containing gas. 
     In one exemplary embodiment, the oxygen-containing gas is at least one gas selected from the group consisting of O 2 , CO, CO 2 , H 2 O, and H 2 O 2 . 
     In one exemplary embodiment, the processing gas further includes a C u H v F w  gas (where u and v are integers equal to or greater than 1, and w is an integer equal to or greater than 0). 
     In one exemplary embodiment, the C u H v F w  gas is at least one gas selected from the group consisting of CH 2 F 2  gas, CHF 3  gas, CH 3 F gas, C 4 H 2 F 6  gas, C 3 H 2 F 4  gas, and CH 4  gas. 
     In one exemplary embodiment, the bias DC signal is a voltage pulse comprising two alternating periods with different voltage levels. 
     In one exemplary embodiment, (b) includes: (b1) forming a plasma from a processing gas containing HF gas and at least one of C x F y  gas and phosphorus-containing gas at a first flow rate ratio to etch the film stack; and (b2) forming a plasma from a processing gas containing at least one of HF gas and C x F y  gas and phosphorus-containing gas at a second flow rate ratio different from the first flow rate ratio to etch the film stack. 
     In one exemplary embodiment, (b1) and (b2) are alternately repeated in (b). 
     In one exemplary embodiment, (b) further includes: (b3) forming a plasma from a processing gas containing HF gas and at least one of C x F y  gas and phosphorus-containing gas at a third flow rate ratio different from the first flow rate ratio and the second flow rate ratio to etch the film stack. 
     In one exemplary embodiment, (b) includes: (b4) forming a plasma from a processing gas containing HF gas, at least one of C x F y  gas and phosphorus-containing gas, and C u H v F w  gas at a fourth flow rate ratio to etch the film stack; and (b5) forming a plasma from a processing gas containing HF gas, at least one of C x F y  gas and phosphorus-containing gas, and C u H v F w  gas at a fifth flow rate ratio different from the fourth flow rate ratio to etch the film stack. 
     In one exemplary embodiment, (b4) and (b5) are alternately repeated. 
     In one exemplary embodiment, (b) further includes: (b6) forming a plasma from a processing gas containing HF gas, at least one of C x F y  gas and phosphorus-containing gas, and C u H v F w  gas at a sixth flow rate ratio different from the fourth flow rate ratio and the fifth flow rate ratio to etch the film stack. 
     Another exemplary embodiment is a plasma processing method performed in a plasma processing apparatus having a chamber, the method comprising: (a) providing a substrate having a film stack including a silicon oxide film and a silicon nitride film to a substrate support in the chamber; (b) forming a plasma in the chamber; and (c) etching the film stack with HF species, C x F y  species (where x and y are integers equal to or greater than 1), and phosphorus active species contained in the plasma, wherein, in (b), the substrate support is controlled to a temperature of 0° C. or more and 70° C. or less, and a bias RF signal of 10 kW or more or a bias DC signal of 4 kV or more is supplied to the substrate support. 
     Another exemplary embodiment is a plasma processing system comprising a chamber, a substrate support disposed in the chamber, a power supply, and a controller, wherein the controller is configured to cause: (a) disposing a substrate having a film stack including a silicon oxide film and a silicon nitride film to a substrate support in the chamber; and (b) forming a plasma from a processing gas containing HF gas and at least one of C x F y  gas (where x and y are integers equal to or greater than 1) and phosphorus-containing gas to etch the film stack, and wherein, in (b), the substrate support is controlled to a temperature of 0° C. or more and 70° C. or less, and a bias RF signal of 10 kW or more or a bias DC signal of 4 kV or more is supplied to the substrate support from the power supply. 
     The following is a detailed description of embodiments of the present disclosure with reference to the drawings. In the drawings, identical or similar elements are denoted by the same reference numbers and redundant descriptions of these elements has been omitted. In the following description, positional relationships such as up, down, left and right are based on the positional relationships shown in the drawings except where otherwise specified. The dimensional ratios in the drawings do not indicate actual ratios, and the actual ratios are not limited to the ratios shown in the drawings. 
     &lt;Example of Plasma Processing System Configuration&gt; 
     An example of a configuration for the plasma processing system will now be described.  FIG.  1    is a figure used to describe an example of a configuration for a capacitively coupled plasma processing apparatus. 
     The plasma processing system includes a capacitively coupled plasma processing apparatus  1  and a controller  2 . The capacitively coupled plasma processing apparatus  1  includes a plasma processing chamber  10 , a gas supply  20 , a power supply  30 , and an exhaust system  40 . The plasma processing apparatus  1  also includes a substrate support  11  and a gas introducer. The gas introducer is configured to introduce at least one processing gas to the plasma processing chamber  10 . The gas introducer includes a shower head  13 . The substrate support  11  is arranged inside the plasma processing chamber  10 . The shower head  13  is arranged above the substrate support  11 . In an exemplary embodiment, the shower head  13  constitutes at least a portion of the ceiling of the plasma processing chamber  10 . The plasma processing chamber  10  has a plasma processing space  10   s  defined by the shower head  13 , the side walls  10   a  of the plasma processing chamber  10 , and the substrate support  11 . The plasma processing chamber  10  has at least one gas supply port for supplying at least one processing gas to the plasma processing space  10   s , and at least one gas exhaust port for exhausting gas from the plasma processing space. The plasma processing chamber  10  is grounded. The shower head  13  and the substrate support  11  are electrically isolated from the plasma processing chamber  10 . 
     The substrate support  11  includes a main body  111  and a ring assembly  112 . The main body  111  has a central region  111   a  for supporting a substrate W and an annular region  111   b  for supporting the ring assembly  112 . A wafer is an example of a substrate W. The annular region  111   b  of the main body  111  surrounds the central region  111   a  of the main body  111  in a plan view. The substrate W is arranged in the central region  111   a  of the main body  111 , and the ring assembly  112  is arranged in the annular region  111   b  of the main body  111  so as to surround the substrate W in the central region  111   a  of the main body  111 . Therefore, the central region  111   a  is also known as the substrate support surface for supporting the substrate W, and the annular region  111   b  is known as the ring support surface for supporting the ring assembly  112 . 
     In one embodiment, the main body  111  includes a base  1110  and an electrostatic chuck  1111 . The base  1110  includes a conductive member. The conductive member of the base  1110  can function as a lower electrode. The electrostatic chuck  1111  is arranged on the base  1110 . The electrostatic chuck  1111  includes a ceramic member  1111   a  and an electrostatic electrode  1111   b  disposed within the ceramic member  1111   a . The ceramic member  1111   a  has a central region  111   a . In one embodiment, the ceramic member  1111   a  also has an annular region  111   b . Note that another member surrounding the electrostatic chuck  1111 , such as an annular electrostatic chuck or an annular insulating member, may have the annular region  111   b . In this case, the ring assembly  112  may be disposed on the annular electrostatic chuck or the annular insulating member, or may be disposed on both the electrostatic chuck  1111  and the annular insulating member. An RF or DC electrode may also be placed within the ceramic member  1111   a , in which case the RF or DC electrode functions as the lower electrode. An RF or DC electrode is also referred to as a bias electrode if bias RF signals or DC signals described below are connected to the RF or DC electrode. Note that both the conductive member of the base  1110  and the RF or DC electrode may function as two lower electrodes. 
     The ring assembly  112  includes one or more annular members. In one embodiment, the one or more annular members include one or more edge rings and at least one cover ring. The edge ring or rings is made of a conductive material or an insulating material, and the cover ring is made of an insulating material. 
     Also, the substrate support  11  may include a temperature control module configured to keep at least one of the electrostatic chuck  1111 , the ring assembly  112 , and the substrate at a target temperature. The temperature control module may include a heater, a heat transfer medium, a channel  1110   a , or combinations of these. A heat transfer fluid such as brine or a gas flows through the flow path  1110   a . In one embodiment, channels  1110   a  are formed in the base  1110  and one or more heaters are disposed in the ceramic member  1111   a  of electrostatic chuck  1111 . The substrate support  11  may also include a heat transfer gas supply configured to supply a heat transfer gas between the back surface of the substrate W and the central region  111   a.    
     The shower head  13  is configured to introduce at least one processing gas from the gas supply  20  to the plasma processing space  10   s . The shower head  13  has at least one gas supply port  13   a , at least one gas diffusion chamber  13   b , and multiple gas introduction ports  13   c . The processing gas supplied to the gas supply port  13   a  passes through the gas diffusion chamber  13   b  and is introduced to the plasma processing space  10   s  via the gas introduction ports  13   c . The shower head  13  also includes an upper electrode. In addition to the showerhead  13 , the gas introducer may include one or more side gas injectors (SGI) attached to one or more openings formed in a side wall  10   a.    
     The gas supply  20  may include at least one gas source  21  and at least one flow controller  22 . In one embodiment, the gas supply  20  is configured to supply at least one processing gas from its respective gas source  21  via its respective flow controller  22  to the shower head  13 . Each flow controller  22  may include, for example, a mass flow controller or a pressure-controlled flow controller. The gas supply  20  may also include one or more flow modulating devices that modulate or pulse the flow rate of at least one processing gas. 
     The power supply  30  includes an RF power supply  31  serving as the first power supply coupled to the plasma processing chamber  10  via at least one impedance matching circuit. The RF power supply  31  is configured to supply at least one RF signal (RF power), such as a source RF signal and a bias RF signal, to at least one lower electrode and/or to at least one upper electrode. In this way, a plasma is formed from at least one processing gas supplied to the plasma processing space  10   s . Thus, the RF power supply  31  may function as at least part of a plasma generator configured to form a plasma from one or more processing gases in the plasma processing chamber  10 . Also, by supplying a bias RF signal to at least one lower electrode, a bias potential is generated in the substrate W, and ion components in the formed plasma can be attracted to the substrate W. 
     In one embodiment, the RF power supply  31  includes a first RF generator  31   a  and a second RF generator  31   b . The first RF generator  31   a  is coupled to at least one lower electrode and/or to at least one upper electrode via at least one impedance matching circuit and configured to generate a source RF signal (source RF power) for plasma generation. In one embodiment, the source RF signal has a frequency in the range from 10 MHz to 150 MHz. In one embodiment, the first RF generator  31   a  may be configured to generate multiple source RF signals with different frequencies. One or more source RF signals generated are provided to at least one lower electrode and/or to at least one upper electrode. 
     A second RF generator  31   b  is coupled to the at least one lower electrode via at least one impedance matching circuit and configured to generate a bias RF signal (bias RF power). The frequency of the bias RF signal may be the same as or different from the frequency of the source RF signal. In one embodiment, the bias RF signal has a frequency lower than that of the source RF signal. In one embodiment, the bias RF signal has a frequency in the range from 100 kHz to 60 MHz. In one embodiment, the second RF generator  31   b  may be configured to generate multiple bias RF signals with different frequencies. One or more bias RF signals that have been generated are supplied to at least one lower electrode. Also, in various embodiments, at least one of the source RF signal and the bias RF signal may be pulsed. 
     The power supply  30  may also include a DC power supply  32  coupled to the plasma processing chamber  10 . The DC power supply  32  includes a first DC generator  32   a  and a second DC generator  32   b . In one embodiment, the first DC generator  32   a  is connected to at least one lower electrode and configured to generate a first DC signal. A generated first bias DC signal is supplied to at least one lower electrode. In one embodiment, the second DC generator  32   b  is connected to at least one upper electrode and configured to generate a second DC signal. The generated second DC signal is supplied to at least one upper electrode. 
     In various embodiments, at least one of the first and second DC signals may be pulsed. In this case, a sequence of DC-based voltage pulses is supplied to at least one lower electrode and/or at least one upper electrode. The voltage pulses may have rectangular, trapezoidal or triangular pulse waveforms, or some combination of these pulse waveforms. In one embodiment, a waveform generator for generating a sequence of voltage pulses from a DC signal is connected between the first DC generator  32   a  and at least one lower electrode. Thus, the first DC generator  32   a  and the waveform generator constitute a voltage pulse generator. When a second DC generator  32   b  and a waveform generator constitute a voltage pulse generator, the voltage pulse generator is connected to at least one upper electrode. The voltage pulse may have a positive polarity or a negative polarity. Also, the sequence of voltage pulses may include one or more positive voltage pulses and one or more negative voltage pulses in a single cycle. Note that first and second DC generators  32   a ,  32   b  may be provided in addition to the RF power supply  31 , or a first DC generator  32   a  may be provided instead of a second RF generator  31   b.    
     The exhaust system  40  can be connected, for example, to a gas outlet  10   e  provided at the bottom of the plasma processing chamber  10 . The exhaust system  40  may include a pressure regulating valve and a vacuum pump. The pressure control valve regulates the pressure inside the plasma processing space  10   s . The vacuum pump may include a turbo molecular pump, a dry pump, or a combination of these. 
     The controller  2  processes computer-executable instructions that get the plasma processing apparatus  1  to perform the steps described in the present disclosure. The controller  2  may be configured to get each element in the plasma processing apparatus  1  to perform the steps described in the present specification. In an exemplary embodiment, some or all of the controller  2  may be provided as part of the plasma processing apparatus  1 . The controller  2  may include, for example, a computer  2   a . The computer  2   a  may include, for example, a central processing unit (CPU)  2   a   1 , a storage  2   a   2 , and a communication interface  2   a   3 . The central processing unit  2   a   1  may be configured to perform control operations by retrieving a program from the storage  2   a   2  and executing the retrieved program. This program may be stored in the storage  2   a   2  in advance or may be acquired via another medium when necessary. The acquired program is stored in the storage  2   a   2 , retrieved from the storage  2   a   2  and executed by the central processing unit  2   a   1 . The medium may be any storage medium readable by the computer  2   a  or may be a communication line connected to the communication interface  2   a   3 . The storage  2   a   2  may include random access memory (RAM), read-only memory (ROM), a hard disk drive (HDD), a solid state drive (SSD), or a combination of these. The communication interface  2   a   3  may communicate with other configurations in the plasma processing apparatus  1  via a communication line such as a local area network (LAN). 
     &lt;Example of the Plasma Processing Method&gt; 
       FIG.  2    is a flow chart showing a plasma processing method in an exemplary embodiment (“the processing method” below). As shown in  FIG.  2   , the processing method includes a step ST 1  of providing a substrate and a step ST 2  of etching a film stack on the substrate. The processing in each step may be performed by the plasma processing system in  FIG.  1   . An example will now be described in which the controller  2  controls each element of the plasma processing apparatus  1  to perform the processing method on a substrate W. 
     (Step ST 1 : Supplying a Substrate) 
     In step ST 1 , a substrate W is provided to the plasma processing space  10   s  of the plasma processing apparatus  1 . The substrate W is disposed on the upper surface of the substrate support  11  so as to face the upper electrode, and is held in place on the substrate support  11  by the electrostatic chuck  1111 . 
       FIG.  3    is a figure schematically illustrating an example of the cross-sectional structure of the substrate W provided in step ST 1 . On the substrate W, a film stack LF and a mask film MF are formed on an underlying film UF in this order. The substrate W may be used, for example, in the manufacture of semiconductor devices, including semiconductor memory devices such as DRAMs and 3D-NAND flash memories. 
     The underlying film UF may be, for example, a silicon wafer or an organic film, dielectric film, metal film, or semiconductor film formed on a silicon wafer. The underlying film UF may be configured by stacking multiple films. 
     The film stack LF includes a silicon oxide film (SiO x  film) LF 1  and a silicon nitride film LF 2 . The film stack LF may be multilayer film configured by alternately stacking multiple silicon oxide films LF 1  and silicon nitride films LF 2 . The film stack LF is the film to be etched by the processing method. 
     The underlying film UF and/or the film stack LF may be formed using, for example, the CVD method or the spin coating method. The underlying film UF and/or the film stack LF may be a flat film or may be uneven film. 
     The mask film MF is formed on the film stack LF. The mask film MF defines at least one opening OP on the film stack LF. The opening OP is a space above the film stack LF and is surrounded by side walls of the mask film MF. Specifically, in  FIG.  3   , the upper surface of the film stack LF has a region covered with the mask film MF and a region exposed at the bottom of the opening OP. 
     The opening OP may have any shape when the substrate W is viewed from above, that is, when the substrate W is viewed downward from above in  FIG.  3   . The shape of the opening may be, for example, a circle, an ellipse, a rectangle, a line, or a shape combining one or more of these. The mask film MF may have multiple side walls, and the multiple side walls may define multiple openings OP. The openings OP may each have a linear shape and may be arranged at regular intervals to form a line and space pattern. Alternatively, the openings OP may each have a hole shape and form an array pattern. 
     The mask film MF is, for example, a carbon-containing film. The carbon-containing film may be, for example, an amorphous carbon film, a spin-on carbon film, or a photoresist film. The mask film MF may be a single layer mask consisting of one layer or a multilayer mask consisting of two or more layers. The mask film MF may be formed using CVD method or the spin coating method. The opening OP may be formed by etching the mask film MF. The mask film MF may be formed by lithography. 
     At least a portion of the process of forming each structure on the substrate W may be performed in the plasma processing chamber  10 . In one example, the step of etching the mask film MF to form an opening OP may be performed in the plasma processing chamber  10 . In other words, the etching of the opening OP and the etching of the film stack LF to be described later may be continuously performed in the same chamber. Also, the substrate W may be carried into the plasma processing space  10   s  of the plasma processing apparatus  1  and placed on the upper surface of the substrate support  11  after some or all of each structure on the substrate W has been formed in an apparatus or chamber outside of the plasma processing apparatus  1 . 
     (Step ST 2 : Etching the Film Stack LF) 
     In step ST 2 , the film stack LF is etched. Step ST 2  includes a step ST 21  of setting the temperature of the substrate support, a step ST 22  of supplying a processing gas, and a step ST 23  of forming plasma. 
     In step ST 21 , a temperature control module adjusts the temperature of the substrate support  11  to a set temperature of 0° C. or higher and 70° C. or lower. The set temperature may be 1° C. or higher and 60° C. or lower, or may be 30° C. or higher and 60° C. or lower. The temperature of the substrate support  11  is kept at the set temperature during processing after step ST 2 . In one example, adjusting or maintaining the temperature of the substrate support  11  includes adjusting or maintaining the temperature of the heat transfer fluid flowing through a channel  1110   a  to a set temperature. Adjusting or maintaining the temperature of the substrate support  11  also includes adjusting or maintaining the temperature of the heat transfer fluid flowing through the channel  1110   a  at a temperature different from the set temperature so that the temperature of the substrate support  11  reaches the set temperature. Note that the heat transfer fluid may start to flow in the channel  1110   a  before or after the substrate W has been placed on the substrate support, or at the same time. In another example, adjusting or maintaining the temperature of the substrate support  11  includes adjusting or maintaining the temperature of the substrate W at a set temperature. Adjusting or maintaining the temperature of the substrate support  11  includes adjusting or maintaining the temperature of the substrate W at a temperature different from the set temperature so that the temperature of the substrate support  11  reaches the set temperature. “Adjusting” or “maintaining” the temperature also includes inputting, selecting, or storing a temperature in the controller  2 . 
     In step ST 22 , a processing gas is supplied to the plasma processing space  10   s . The processing gas contains HF gas. The processing gas may contain, for example, 50 vol % or more, 60 vol % or more, or 70 vol % or more of HF gas relative to the volume of the processing gas. The HF gas can be gas with a high degree of purity, for example, a purity of 99.999% or more. The processing gas includes C x F y  gas (where x and y are integers equal to or greater than 1) or phosphorus-containing gas in addition to HF gas. The processing gas may contain both C x F y  gas and a phosphorus-containing gas. The C x F y  gas can contribute to an improvement in the etching rate of the silicon oxide film LF 1 . The phosphorus-containing gas promotes adsorption of the HF gas in the silicon nitride film or silicon oxide film, and can contribute to an improvement in the etching rate of these films. 
     The flow rate ratio of the C x F y  gas or the phosphorus-containing gas relative to the HF gas may be set based on the structure of the film stack LF and the pattern of the mask film MF. For example, the flow rate ratio may be set based on the compositional ratio of the silicon oxide film LF 1  relative to the film stack LF. In one example, the flow rate ratio of the C x F y  gas or the phosphorus-containing gas relative to the HF gas may be increased as the compositional ratio of the silicon oxide film LF 1  relative to the film stack LF increases. Note that the processing gas may include a gas capable of producing HF species in the chamber instead of or in addition to HF gas. HF species include hydrogen fluoride gas, radicals and/or ions. Gases capable of generating HF species include, for example, one or more gases from the group consisting of CH 2 F 2  gas, C 3 H 2 F 4  gas, C 3 H 2 F 6  gas, C 3 H 3 F 5  gas, C 4 H 2 F 6  gas, C 4 H 5 F 5  gas, C 4 H 2 F 8  gas, C 5 H 2 F 6  gas, C 5 H 2 F 10  gas, and C 5 H 3 F 7  gas. In one example, at least one gas selected from the group consisting of CH 2 F 2  gas, C 3 H 2 F 4  gas and C 4 H 2 F 6  gas is used as a gas capable of generating HF species. 
     The C x F y  gas may be at least one gas selected from the group consisting of C 3 F 6  gas, C 4 F 6  gas, C 4 F 8  gas, and C 3 F 8  gas. 
     A phosphorus-containing gas is a gas with phosphorus-containing molecules. The phosphorus-containing molecule may be an oxide such as tetraphosphorus decaoxide (P 4 O 10 ), tetraphosphorus octaoxide (P 4 O 8 ), or tetraphosphorus hexaoxide (P 4 O 6 ). Tetraphosphorus decaoxide is sometimes called diphosphorus pentoxide (P 2 O 5 ). The phosphorus-containing molecule may be a halide (phosphorus halide) such as phosphorus trifluoride (PF 3 ), phosphorus pentafluoride (PF 5 ), phosphorus trichloride (PCl 3 ), phosphorus pentachloride (PCl 5 ), phosphorus tribromide (PBr 3 ), phosphorus pentabromide (PBr 5 ), or phosphorus iodide (PI 3 ). In other words, the phosphorus-containing molecule may contain fluorine as the halogen element, such as phosphorus fluoride. Alternatively, the phosphorus-containing molecule may contain a halogen element other than fluorine as the halogen element. The phosphorus-containing molecule may be a phosphoryl halide such as phosphoryl fluoride (POF 3 ), phosphoryl chloride (POCl 3 ), and phosphoryl bromide (POBr 3 ). The phosphorus-containing molecule may be phosphine (PH 3 ), calcium phosphide (such as Ca 3 P 2 ), phosphoric acid (H 3 PO 4 ), sodium phosphate (Na 3 PO 4 ), or hexafluorophosphate (HPF 6 ). The phosphorus-containing molecule may be a fluorophosphine (H g PF h ). Here, the sum of g and h is 3 or 5. Examples of fluorophosphines include HPF 2  and H 2 PF 3 . The processing gas may contain, at least as a phosphorus-containing molecule, one or more of the phosphorus-containing molecules described above. For example, the processing gas can include at least one of PF 3 , PCl 3 , PF 5 , PCl 5 , POCl 3 , PH 3 , PBr 3 , or PBr 5  as a phosphorus-containing molecule. When each phosphorus-containing molecule in the processing gas is a liquid or solid, each phosphorus-containing molecule can be vaporized by, for example, heating and supplying the gas into the plasma processing space  10   s.    
     The processing gas may also include an oxygen-containing gas. An oxygen-containing gas can suppress clogging of the mask film MF during etching. For example, at least one gas selected from the group consisting of O 2 , CO, CO 2 , H 2 O and H 2 O 2  may be used as the oxygen-containing gas. In one example, the processing gas may include an oxygen-containing gas other than H 2 O, that is, at least one gas selected from the group consisting of O 2 , CO, CO 2 , and H 2 O 2 . The flow rate of the oxygen-containing gas may be adjusted in response to the flow rate of the C x F y  gas. 
     The processing gas may also include a C u H v F w  gas (where u and v are integers equal to or greater than 1 and w is an integer equal to or greater than 0). The C u H v F w  gas may be at least one gas selected from the group consisting of CH 2 F 2  gas, CHF 3  gas, CH 3 F gas, C 4 H 2 F 6  gas, C 3 H 2 F 4  gas, and CH 4  gas. 
     The processing gas may also contain a noble gas such as Ar and Kr. The processing gas may further include NF 3  gas. 
     In step ST 23 , a source RF signal (RF power) is supplied from the first RF generator  31   a  to the lower electrode and/or the upper electrode. This forms a plasma from the processing gas supplied to the plasma processing space  10   s . Also, a bias RF signal is supplied to the lower electrode from the second RF generator  31   b  as a bias signal (power). This generates a bias potential in the substrate W. Active species such as ions and radicals in the formed plasma are attracted to the substrate W, and the film stack LF is etched. In other words, the portion corresponding to the opening OP of the mask film MF is etched in the depth direction (direction from top to bottom in  FIG.  3   ) to form a recessed portion in the film stack LF. The timing for starting supply of the source RF signal and the timing for starting supply of the bias signal may be the same or different. 
     A bias DC signal may be used instead of the bias RF signal as the bias signal (power). In other words, a bias potential may be generated in the substrate W by supplying a negative bias DC signal from the DC generator  32   a  to the lower electrode. The source RF signal and the bias signal may both be continuous waves, or one may be continuous and the other pulsed. 
     When a bias RF signal is used as the bias signal, the effective power value of the bias RF signal is 10 kW or more. The effective value of the power of the bias RF signal may be 30 kW or less. When a negative bias DC signal is used as the bias signal, the absolute value of the voltage of the bias DC signal (or the effective value of the absolute value of the voltage in the case of a pulse wave) is 4 kV or more. The absolute value of the voltage of the bias DC signal (or the effective value of the absolute value of the voltage in the case of a pulse wave) may be 15 kV or less. 
     Experiments conducted to evaluate the processing method are described below. The present disclosure is not limited in any way by the following experiments. 
     Experiment 1 
     In Experiment 1, silicon nitride film, silicon oxide film, and carbon-containing film were each etched by forming plasma in the plasma processing apparatus  1  under the following conditions, and the etching rate was measured. An amorphous carbon film was used as the carbon-containing film. 
     Processing gases: HF gas, Ar gas 
     Temperature set for the substrate support: 10° C. 
     Chamber pressure: 25 mT 
     Source RF signal: 40 MHz/5.5 kW 
     Bias RF signal: 400 kHz/15 kW 
       FIG.  4    is a graph showing the measurement results from Experiment 1. The vertical axis in  FIG.  4    indicates the etching rates (ER) [nm/min] of each of the silicon nitride film, the silicon oxide film, and the carbon-containing film. As shown in  FIG.  4   , the etching rate of the silicon nitride film was sufficiently higher than the etching rate of the carbon-containing film. This indicates that at the temperature set for the processing method, the HF species in the plasma, which compose the etchant, are adsorbed on the silicon nitride film, and reactive ion etching can proceed. In contrast, the etching rate of the silicon oxide film was low, being about the same as the etching rate of the carbon-containing film. This is probably because the HF species in the plasma, which compose the etchant, were not adsorbed on the silicon oxide film, and the etching was primarily due to sputtering. In other words, at the temperature set for the processing method, the HF species in the plasma, which compose the etchant, are less likely to be adsorbed to the silicon oxide film. It can be seen from Experiment 1 that this processing gas is not sufficient to improve the etching rate of film stack including silicon nitride film and silicon oxide film at the temperature set for the processing method. 
     Experiment 2 
     In Experiment 2, the following three processing gas patterns were used in the plasma processing apparatus  1  to form plasma, etch silicon nitride film, silicon oxide film and carbon-containing film, and measure the etching rates. An amorphous carbon film was used as the carbon-containing film. The rest of the conditions were the same as in Experiment 1. Pattern 2 and Pattern 3 are examples of processing gases for the processing method. The flow rate of HF gas in Pattern 3 was twice the flow rate of HF gas in Pattern 2. 
     Pattern 1: C 4 F 8  gas and O 2  gas 
     Pattern 2: C 4 F 8  gas, HF gas, and O 2  gas 
     Pattern 3: C 4 F 8  gas, HF gas, and O 2  gas 
       FIG.  5    is a graph showing the measurement results from Experiment 2. The vertical axis in  FIG.  5    indicates the etching rates (ER) [nm/min] of each of the silicon nitride film, the silicon oxide film, and the carbon-containing film in Pattern 1 to Pattern 3. As shown in  FIG.  5   , the etching rate of the silicon oxide film was sufficiently high relative to the carbon-containing film in all of Patterns 1 to 3. The etching rate of the silicon nitride film was lower than that of the carbon-containing film when the processing gas did not contain HF gas (Pattern 1), but increased sharply and became sufficiently higher than that of the carbon-containing film when HF gas was included. The etching rate of the carbon-containing film changed very little even when the flow rate of the HF gas was doubled (Patterns 2 and 3), and was low in all of the Patterns 1 to 3. It can be seen from Experiment 2 that when using C 4 F 8  gas in addition to HF gas as a processing gas at the temperature set for the processing method, selectivity relative to the carbon-containing film can be improved while increasing the etching rate of both the silicon oxide film and the silicon nitride film. 
     Experiment 3 
     In Experiment 3, plasma was formed in the plasma processing apparatus  1  using the following three patterns of processing gas, silicon nitride film, silicon oxide film, and carbon-containing film were etched, and the etching rates were measured. An amorphous carbon film was used as the carbon-containing film. The rest of the conditions were the same as in Experiment 1. Pattern 6 is an example of a processing gas for the processing method. 
     Pattern 4: C 4 F 8  gas, CH 2 F 2  gas HF gas, and O 2  gas 
     Pattern 5: C 4 F 8  gas, C 3 H 2 F 4  gas, and O 2  gas 
     Pattern 6: C 4 F 8  gas, HF gas, and O 2  gas 
       FIG.  6    is a graph showing the measurement results from Experiment 3. The vertical axis in  FIG.  6    indicates the etching rates (ER) [nm/min] of each of the silicon nitride film, the silicon oxide film, and the carbon-containing film in Pattern 4 to Pattern 6. As shown in  FIG.  6   , the etching rate of the silicon oxide film and the silicon nitride film was improved by about 10 to 20% in the case of Pattern 6 containing HF gas as a processing gas compared to Patterns 4 and 5 which contained CH 2 F 2  gas and C 3 H 2 F 4  gas as a processing gas. The etching rate of the carbon-containing film in Pattern 6 was about the same as that in Patterns 4 and 5. It can be seen from Experiment 3 that when using HF gas as a processing gas at the temperature set for the processing method, high selectivity can be maintained relative to the carbon-containing film while increasing the etching rate compared to the cases using CH 2 F 2  gas or C 3 H 2 F 4  gas. 
     Experiment 4 
     In Experiment 4, plasma was formed using HF gas and a phosphorus-containing gas in plasma processing apparatus  1  to etch silicon nitride film, silicon oxide film, and carbon-containing film, and measure the etching rates. An amorphous carbon film was used as the carbon-containing film. PF 3  gas was used as the phosphorus-containing gas, and the etching rate was measured by changing the flow rate ratio (vol %) relative to the total volume of processing gas to 0%, 2%, 7%, 11%, 20%, and 26%. The rest of the conditions were the same as in Experiment 1. 
       FIG.  7    is a graph showing the measurement results from Experiment 3. The horizontal axis in  FIG.  7    indicates the flow rate ratio [vol %] of the phosphorus-containing gas in the processing gas. The vertical axis indicates the etching rate (ER) [nm/min]. As shown in  FIG.  7   , the etching rates of the silicon oxide film and the silicon nitride film increased significantly as the flow rate ratio of the phosphorus-containing gas in the processing gas increased. The etching rate when the processing gas contained 26% of the phosphorus-containing gas was 5.6 times higher for the silicon oxide film and 2.5 times higher for the silicon nitride film than when the processing gas did not contain a phosphorus-containing gas (0 vol %). This is probably because the phosphorus-containing gas accelerated adsorption of the HF gas, which is an etchant, to the silicon nitride film and the silicon oxide film. Also, as shown in  FIG.  7   , the etching rate of the carbon-containing film did not increase when the flow rate of the phosphorus-containing gas in the processing gas was increased. It can be seen from Experiment 4 that when using a phosphorus-containing gas in addition to HF gas as a processing gas at the temperature set for the processing method, selectivity relative to the carbon-containing film can be improved while increasing the etching rate of both the silicon oxide film and the silicon nitride film. 
     EXAMPLES 
     Examples of the processing method will now be described. The present disclosure is not limited in any way by the following examples. 
     Examples 1 and 2 
     The processing method was applied using the plasma processing apparatus  1  to etch the film stack LF on the substrate W shown in  FIG.  3   . In Examples 1 and 2, HF gas, PF 3  gas, and Ar gas were used as processing gases. The flow rate ratio (vol %) of the phosphorus-containing gas to the processing gas was 13% in Example 1 and 20% in Example 2. The other conditions in both Example 1 and Example 2 were as follows. 
     Temperature set for the substrate support: 10° C. 
     Chamber pressure: 25 mT 
     Source RF signal: 40 MHz/5.5 kW 
     Bias RF signal: 400 kHz/15 kW 
     Reference Example 1 
     In the reference example, the film stack LF on the substrate W was etched under the same conditions as in the examples, except that the processing gas was changed as follows. 
     Reference Example 1: HF gas, Ar gas 
     The etching rates for the film stacks LF in Examples 1 and 2 were 512 [nm/min] and  518  [nm/min], respectively. In contrast, the etching rate in Reference Example 1 was 231 [nm/min]. In other words, the etching rate for the film stack LF in both Example 1 and Example 2 was significantly better than that in Reference Example 1. 
     In the processing method, C x F y  gas or phosphorus-containing gas is used as a processing gas in addition to HF gas. As a result, the etching rate of both the silicon oxide film LF1 and the silicon nitride film LF2 constituting the film stack LF can be improved, and the selectivity relative to the mask film MF can be improved. 
     Also, in the processing method, a bias RF signal of 10 kW or more or a bias DC signal of 4 kV or more is supplied as a bias signal during etching. By using a high-power bias RF signal, volatilization of reaction by-products caused by etching of the film stack LF can be promoted. For example, volatilization of reaction by-products composed primarily of ammonium silicofluoride (AFS) that are generated during etching of silicon nitride film LF2 can be accelerated. This can improve the etching rate of the film stack LF. 
     Modified Example of the Method 
       FIG.  8    is a flowchart showing a modified example of the processing method. As shown in  FIG.  8   , the processing method up to step ST 21  is the same in the modified example but differs from the processing method described above after step ST 21 . In the modified example, step ST 2 A of etching the film stack LF, after step ST 21  of adjusting the temperature of the substrate supporting, has step ST 22   a  of supplying a first processing gas, step ST 23   a  of forming a plasma from the first processing gas, step ST 22   b  of supplying a second processing gas, step ST 23   b  of forming a plasma from the second processing gas, and step ST 24  of determining whether the etching has been completed. In step ST 2 A, steps ST 22   a  to ST 23   b  are repeated until it has determined in step ST 24  that the etching has been completed. 
     The first processing gas and the second processing gas contain HF gas and at least one of C x F y  gas and phosphorus-containing gas. The first processing gas and the second processing gas may also contain other gases such as C u H v F w  gas and oxygen-containing gas as in the processing method. 
     The first processing gas and the second processing gas may differ in terms of the composition and flow rate of the processing gas. In one example, the first processing gas may contain HF gas and at least one of C x F y  gas and phosphorus-containing gas at a first flow rate ratio. The second processing gas may contain HF gas and at least one of a C x F y  gas and a phosphorus-containing gas at a second flow rate ratio different from the first flow rate ratio. 
     In one example, the first processing gas may contain HF gas, at least one of C x F y  gas and phosphorus-containing gas, and C u H v F w  gas at a fourth flow rate ratio. The second processing gas may contain HF gas, at least one of C x F y  gas and phosphorus-containing gas, and C u H v F w  gas at a fifth flow rate ratio different from the fourth flow rate ratio. 
     The modified example may also include a step of supplying a third processing gas and a step of forming a plasma from the third processing gas between steps ST 23   b  and ST 24 . The third processing gas may contain HF gas and at least one of a C x F y  gas and a phosphorus-containing gas at a third flow rate ratio different from the first flow rate ratio and the second flow rate ratio. Also, the third processing gas may contain HF gas, at least one of a C x F y  gas and a phosphorus-containing gas, and a C u H v F w  gas at a sixth flow rate ratio different from the fourth flow rate ratio and the fifth flow rate ratio. 
     Also, in the modified example, the steps using the first processing gas, the second processing gas, and the third processing gas may be performed once without including step ST 24 . 
     In the modified example, the composition of the processing gas can be optimized according to, for example, the compositional ratio of the silicon oxide film LF 1  and the silicon nitride film LF 2  that constitute the film stack LF and the aspect ratio of the film stack LF. This can improve the overall etching rate for the film stack LF. 
     Various modifications may be made to the processing method without departing from the scope and spirit of this disclosure. For example, the processing method may be performed using a plasma processing apparatus using a plasma source other than that of a capacitively coupled plasma processing apparatus  1 , such as inductively coupled plasma or microwave plasma. 
     In one exemplary embodiment of the present disclosure, the etching rate can be improved. 
     The embodiments described above were provided for illustrative purposes only and are not intended to limit the scope of the present disclosure. Various modifications may be applied to each of these embodiments without departing from the scope and spirit of the present disclosure. For example, some elements in one embodiment can be added to another embodiment. Also, some elements in one embodiment can be replaced with corresponding elements from another embodiment.