Plasma processing method and plasma processing system

Abstract

To provide a plasma processing method and a plasma processing system which enable suppression of necking in etching.SOLUTION: The method comprises the following steps: ST1 of supplying a substrate having a first region where an etching film is exposed and a second region where a mask film is exposed; ST2 of supplying a process gas including a carbon-containing gas into a chamber to generate plasma from the process gas, thereby etching the etching film and concurrently, forming a protection film on the mask film; and ST3 of supplying a process gas into the chamber to generate plasma from the process gas to further etch the etching film and to remove at least part of the protection film. The step ST2 includes a first period and a second period, and a flow rate of the carbon-containing gas in the first period is larger than a flow rate of the carbon-containing gas in the second period. The step ST3 includes a third period and a fourth period, and a flow rate of the carbon-containing gas in the third period is smaller than the flow rate of the carbon-containing gas in the second period and a flow rate of the carbon-containing gas in the fourth period.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] SUMMARY Exemplary embodiments of the present disclosure relate to a plasma processing method and a plasma processing system. [Background technology]

[0002] Patent Document 1 discloses a technique for improving the selectivity to a mask film. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] International Publication No. 2014 / 046083 Summary of the Invention [Problem to be solved by the invention]

[0004] The present disclosure provides techniques for suppressing necking during etching. [Means for solving the problem]

[0005] In one exemplary embodiment of the present disclosure, a plasma processing method is provided that is performed in a plasma processing apparatus having a chamber. The plasma processing method includes: (a) providing a substrate having an etching film and a mask film into the chamber, the substrate having a first region where the etching film is exposed and a second region where the mask film is exposed; (b) supplying a process gas containing a carbon-containing gas into the chamber to generate plasma from the process gas to etch the etching film and form a protective film on the mask film; and (c) supplying the process gas into the chamber to generate plasma from the process gas to further etch the etching film and remove at least a portion of the protective film, wherein the process (b) includes a first period and a second period, and a flow rate of the carbon-containing gas in the first period is greater than a flow rate of the carbon-containing gas in the second period; and the process (c) includes a third period and a fourth period, and a flow rate of the carbon-containing gas in the third period is less than a flow rate of the carbon-containing gas in the second period and a flow rate of the carbon-containing gas in the fourth period. [Effects of the Invention]

[0006] According to one exemplary embodiment of the present disclosure, a technique for suppressing necking during etching can be provided. [Brief explanation of the drawings]

[0007] [Figure 1] 1 is a diagram illustrating an exemplary plasma processing apparatus system. [Figure 2] 1 is a flowchart showing the present processing method. [Figure 3] 2 is a diagram schematically illustrating an example of a cross-sectional structure of a substrate W provided in step ST1. FIG. [Figure 4A] 10 is a diagram schematically illustrating an example of a cross-sectional structure of the substrate W after processing in the first period of step ST2. FIG. [Figure 4B] 10 is a diagram schematically illustrating an example of a cross-sectional structure of the substrate W after processing in a second period of step ST2. FIG. [Figure 5A] 10 is a diagram schematically illustrating an example of a cross-sectional structure of the substrate W after processing in a third period of step ST3. FIG. [Figure 5B] 10 is a diagram schematically illustrating an example of a cross-sectional structure of the substrate W after processing in a fourth period of step ST3. FIG. [Figure 6A] 10 is a timing chart showing an example of the flow rate of the carbon-containing gas in steps ST2 and ST3. [Figure 6B] 10 is a timing chart showing an example of the flow rate of the oxygen-containing gas in steps ST2 and ST3. [Figure 6C] 10 is a timing chart showing an example of bias voltages in steps ST2 and ST3. [Figure 7A] 10 is a timing chart showing another example of the flow rate of the carbon-containing gas in steps ST2 and ST3. [Figure 7B] 10 is a timing chart showing another example of the flow rate of the oxygen-containing gas in steps ST2 and ST3. [Figure 7C] 10 is a timing chart showing another example of the bias voltage in steps ST2 and ST3. [Figure 8] 10 is a diagram schematically showing an example of a cross-sectional structure of a substrate W in another example of the present processing method. FIG. DETAILED DESCRIPTION OF THE INVENTION

[0008] Hereinafter, each embodiment of the present disclosure will be described.

[0009] In one exemplary embodiment, a plasma processing method is provided that is performed in a plasma processing apparatus having a chamber, the plasma processing method comprising: (a) providing a substrate having an etching film and a mask film into the chamber, the substrate having a first region where the etching film is exposed and a second region where the mask film is exposed; (b) supplying a process gas including a carbon-containing gas into the chamber to generate plasma from the process gas to etch the etching film and form a protective film on the mask film; and (c) supplying a process gas into the chamber to generate plasma from the process gas to further etch the etching film and remove at least a portion of the protective film, wherein the step (b) includes a first period and a second period, the flow rate of the carbon-containing gas in the first period being greater than the flow rate of the carbon-containing gas in the second period, and the step (c) includes a third period and a fourth period, the flow rate of the carbon-containing gas in the third period being less than the flow rate of the carbon-containing gas in the second period and the flow rate of the carbon-containing gas in the fourth period.

[0010] In one exemplary embodiment, the mask film is provided on the etching film, and the mask film has an upper surface that covers the etching film in the second region and a side surface that defines an opening that exposes the etching film in the first region, step (b) includes etching the etching film to form a recess in the etching film in the first region and forming a protective film on the upper surface and side surface of the mask film and the recess, and step (c) includes removing at least a portion of the protective film.

[0011] In one exemplary embodiment, the flow rate of the carbon-containing gas during the fourth time period is less than the flow rate of the carbon-containing gas during the second time period.

[0012] In one exemplary embodiment, the flow rate of the carbon-containing gas is changed continuously or stepwise when transitioning from one period to another of the first period to the fourth period.

[0013] In one exemplary embodiment, the plasma processing apparatus further includes a substrate support within the chamber, and (a) includes providing a substrate on the substrate support, and (b) and (c) include supplying a bias signal to the substrate support.

[0014] In one exemplary embodiment, in (b) and (c), the effective value of the power of the bias signal is constant.

[0015] In one exemplary embodiment, in (b), the effective value of the power of the bias signal in the first time period is greater than the effective value of the power of the bias signal in the second time period.

[0016] In one exemplary embodiment, in (c), the effective value of the power of the bias signal in the third period is less than the effective value of the power of the bias signal in the fourth period.

[0017] In one exemplary embodiment, when transitioning from one period to another of the first to fourth periods, the effective value of the power of the bias signal changes continuously or stepwise.

[0018] In one exemplary embodiment, (b) includes supplying a source RF signal to the chamber to generate the plasma, wherein the rms value of the power of the source RF signal during the first time period is less than the rms value of the power of the source RF signal during the second time period.

[0019] In one exemplary embodiment, (c) includes supplying a source RF signal to the chamber to generate the plasma, wherein the effective value of the power of the source RF signal during the third time period is greater than the effective value of the power of the source RF signal during the fourth time period.

[0020] In one exemplary embodiment, the process gas includes an oxygen-containing gas, and in (b), the flow rate of the oxygen-containing gas during the first period is greater than the flow rate of the oxygen-containing gas during the second period.

[0021] In one exemplary embodiment, in (c), the flow rate of the oxygen-containing gas during the third period is less than the flow rate of the oxygen-containing gas during the fourth period.

[0022] In one exemplary embodiment, the substrate further comprises an undercoat film, the etching film being provided in a first region, and the mask film being provided in a second region so as to be adjacent to the etching film in a direction perpendicular to the etching direction of the etching film.

[0023] In one exemplary embodiment, the etching film is a silicon oxide film or a silicon nitride film.

[0024] In one exemplary embodiment, the mask film comprises at least one of a boron-containing silicon film, a carbon-containing film, a nitrogen-containing film, and a tungsten silicon film.

[0025] In one exemplary embodiment, the carbon-containing gas is C a F b (a and b are integers of 1 or more) Gas or C c H d F e (c, d, and e are integers of 1 or greater) gas.

[0026] In one exemplary embodiment, steps (b) and (c) are repeated multiple times.

[0027] In one exemplary embodiment, during at least a portion of the first to fourth periods, the top of the protective film formed on the side of the mask film shifts in the direction in which the etching film is etched as the etching of the etching film progresses.

[0028] In one exemplary embodiment, a plasma processing method is provided that is performed in a plasma processing apparatus having a chamber. The plasma processing method includes: (a) providing a substrate having a first region including an exposed portion of the etching film and a second region including an exposed portion of the mask film into a chamber; (b) a first step of supplying a processing gas containing a carbon-containing gas at a first flow rate into the chamber to generate plasma, thereby etching the etching film and forming a protective film on the mask film; (c) a second step of supplying a processing gas containing a carbon-containing gas at a second flow rate greater than the first flow rate into the chamber to generate plasma, thereby further etching the etching film and forming a protective film on the mask film; (d) a third step of supplying a processing gas containing a carbon-containing gas at a third flow rate into the chamber to generate plasma, thereby further etching the etching film and removing the protective film on the mask film; and (e) a fourth step of supplying a carbon-containing gas at a fourth flow rate less than the second flow rate but greater than the third flow rate into the chamber to generate plasma, thereby further etching the etching film and further removing the protective film on the mask film.

[0029] In one exemplary embodiment, a plasma processing system is provided having a chamber, a process gas supply, a plasma generator, and a controller. In a plasma processing system, a control unit performs control to (a) provide a substrate having an etching film and a mask film into a chamber; (b) a processing gas supply unit supplies a processing gas containing a carbon-containing gas into the chamber, and a plasma generating unit generates plasma from the processing gas to etch the etching film and form a protective film on the mask film; and (c) a processing gas supply unit supplies the processing gas into the chamber, and the plasma generating unit generates plasma from the processing gas to further etch the etching film and remove at least a portion of the protective film, wherein the substrate has a first region where the etching film is exposed and a second region where the mask film is exposed, and the process (b) includes a first period and a second period, and a flow rate of the carbon-containing gas in the first period is greater than a flow rate of the carbon-containing gas in the second period, and the process (c) includes a third period and a fourth period, and a flow rate of the carbon-containing gas in the third period is less than a flow rate of the carbon-containing gas in the second period and a flow rate of the carbon-containing gas in the fourth period.

[0030] <Configuration example of plasma processing system> An example of the configuration of a plasma processing system will be described below: Fig. 1 is a diagram illustrating an example of the configuration of a capacitively coupled plasma processing apparatus.

[0031] The plasma processing system includes a capacitively coupled plasma processing device 1 and a controller 2. The capacitively coupled plasma processing device 1 includes a plasma processing chamber 10, a gas supply unit 20, a power supply 30, and an exhaust system 40. The plasma processing device 1 also includes a substrate support 11 and a gas inlet. The gas inlet is configured to introduce at least one process gas into the plasma processing chamber 10. The gas inlet includes a showerhead 13. The substrate support 11 is disposed within the plasma processing chamber 10. The showerhead 13 is disposed above the substrate support 11. In one embodiment, the showerhead 13 forms at least a portion of the ceiling of the plasma processing chamber 10. The plasma processing chamber 10 has a plasma processing space 10s defined by the showerhead 13, a sidewall 10a of the plasma processing chamber 10, and the substrate support 11. The plasma processing chamber 10 has at least one gas inlet for supplying at least one process gas to the plasma processing space 10s and at least one gas outlet for exhausting gas from the plasma processing space 10s. The plasma processing chamber 10 is grounded. The showerhead 13 and the substrate support 11 are electrically isolated from the plasma processing chamber 10 enclosure.

[0032] The substrate support 11 includes a main body 111 and a ring assembly 112. The main body 111 has a central region 111a for supporting a substrate W and an annular region 111b for supporting the ring assembly 112. A wafer is an example of a substrate W. The annular region 111b of the main body 111 surrounds the central region 111a of the main body 111 in a plan view. The substrate W is disposed on the central region 111a of the main body 111, and the ring assembly 112 is disposed on the annular region 111b of the main body 111 so as to surround the substrate W on the central region 111a of the main body 111. Therefore, the central region 111a is also called a substrate support surface for supporting the substrate W, and the annular region 111b is also called a ring support surface for supporting the ring assembly 112.

[0033] 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 disposed on the base 1110. The electrostatic chuck 1111 includes a ceramic member 1111a and an electrostatic electrode 1111b disposed within the ceramic member 1111a. The ceramic member 1111a has a central region 111a. In one embodiment, the ceramic member 1111a also has an annular region 111b. Note that the annular region 111b may also be provided by another member surrounding the electrostatic chuck 1111, such as an annular electrostatic chuck or an annular insulating member. 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. Furthermore, at least one RF / DC electrode coupled to an RF (Radio Frequency) power supply 31 and / or a DC (Direct Current) power supply 32 (described later) may be disposed within the ceramic member 1111a. In this case, the at least one RF / DC electrode functions as a lower electrode. When a bias RF signal and / or a DC signal (described later) is supplied to the at least one RF / DC electrode, the RF / DC electrode is also called a bias electrode. Note that the conductive member of the base 1110 and the at least one RF / DC electrode may function as multiple lower electrodes. Alternatively, the electrostatic electrode 1111b may function as the lower electrode. Therefore, the substrate support 11 includes at least one lower electrode.

[0034] 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 rings are formed of a conductive or insulating material, and the cover rings are formed of an insulating material.

[0035] The substrate support 11 may also include a temperature adjustment module configured to adjust at least one of the electrostatic chuck 1111, the ring assembly 112, and the substrate to a target temperature. The temperature adjustment module may include a heater, a heat transfer medium, a flow passage 1110a, or a combination thereof. A heat transfer fluid such as brine or a gas flows through the flow passage 1110a. In one embodiment, the flow passage 1110a is formed in the base 1110, and one or more heaters are disposed in the ceramic member 1111a of the electrostatic chuck 1111. The substrate support 11 may also include a heat transfer gas supply configured to supply a heat transfer gas to a gap between the backside of the substrate W and the central region 111a.

[0036] The showerhead 13 is configured to introduce at least one processing gas from the gas supply unit 20 into the plasma processing space 10s. The showerhead 13 has at least one gas supply port 13a, at least one gas diffusion chamber 13b, and multiple gas inlets 13c. The processing gas supplied to the gas supply port 13a passes through the gas diffusion chamber 13b and is introduced into the plasma processing space 10s from the multiple gas inlets 13c. The showerhead 13 also includes at least one upper electrode. In addition to the showerhead 13, the gas introduction unit may also include one or more side gas injectors (SGIs) attached to one or more openings formed in the sidewall 10a.

[0037] The gas supply unit 20 may include at least one gas source 21 and at least one flow controller 22. In one embodiment, the gas supply unit 20 is configured to supply at least one process gas from a corresponding gas source 21 to the showerhead 13 via a corresponding flow controller 22. Each flow controller 22 may include, for example, a mass flow controller or a pressure-controlled flow controller. Additionally, the gas supply unit 20 may include one or more flow modulation devices to modulate or pulse the flow rate of the at least one process gas.

[0038] The power supply 30 includes an RF power supply 31 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) to at least one lower electrode and / or at least one upper electrode. This generates a plasma from at least one process gas supplied to the plasma processing space 10s. Therefore, the RF power supply 31 can function as at least a part of a plasma generating unit configured to generate a plasma from one or more process gases in the plasma processing chamber 10. In addition, by supplying a bias RF signal (bias signal) to the at least one lower electrode, a bias potential (bias power) is generated on the substrate W, thereby attracting ion components in the formed plasma to the substrate W.

[0039] In one embodiment, the RF power supply 31 includes a first RF generating unit 31a and a second RF generating unit 31b. The first RF generating unit 31a is coupled to at least one lower electrode and / or at least one upper electrode via at least one impedance matching circuit and is 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 of 10 MHz to 150 MHz. In one embodiment, the first RF generating unit 31a may be configured to generate multiple source RF signals having different frequencies. The generated one or more source RF signals are supplied to at least one lower electrode and / or at least one upper electrode.

[0040] The second RF generating unit 31b is coupled to 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 the frequency of the source RF signal. In one embodiment, the bias RF signal has a frequency in the range of 100 kHz to 60 MHz. In one embodiment, the second RF generating unit 31b may be configured to generate multiple bias RF signals having different frequencies. The generated one or more bias RF signals are supplied to at least one lower electrode. In various embodiments, at least one of the source RF signal and the bias RF signal may be pulsed.

[0041] 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 32a and a second DC generator 32b. In one embodiment, the first DC generator 32a is connected to at least one lower electrode and configured to generate a first DC signal. The generated first bias DC signal is applied to the at least one lower electrode. In one embodiment, the second DC generator 32b is connected to at least one upper electrode and configured to generate a second DC signal. The generated second DC signal is applied to the at least one upper electrode.

[0042] In various embodiments, at least one of the first and second DC signals may be pulsed. In this case, a sequence of voltage pulses is applied to at least one lower electrode and / or at least one upper electrode. The voltage pulses may have a rectangular, trapezoidal, triangular, or combination thereof pulse waveform. In one embodiment, a waveform generator for generating a sequence of voltage pulses from the DC signal is connected between the first DC generator 32a and at least one lower electrode. Thus, the first DC generator 32a and the waveform generator constitute a voltage pulse generator. When the second DC generator 32b and the waveform generator constitute a voltage pulse generator, the voltage pulse generator is connected to at least one upper electrode. The voltage pulses may have either positive or negative polarity. Furthermore, the sequence of voltage pulses may include one or more positive voltage pulses and one or more negative voltage pulses within one period. The first and second DC generating units 32a and 32b may be provided in addition to the RF power supply 31, or the first DC generating unit 32a may be provided instead of the second RF generating unit 31b.

[0043] The exhaust system 40 may be connected to, for example, a gas exhaust port 10e 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 regulating valve regulates the pressure in the plasma processing space 10s. The vacuum pump may include a turbomolecular pump, a dry pump, or a combination thereof.

[0044] The control unit 2 processes computer-executable instructions that cause the plasma processing apparatus 1 to perform various processes described in this disclosure. The control unit 2 may be configured to control each element of the plasma processing apparatus 1 to perform various processes described herein. In one embodiment, part or all of the control unit 2 may be included in the plasma processing apparatus 1. The control unit 2 may include a processing unit 2a1, a storage unit 2a2, and a communication interface 2a3. The control unit 2 may be implemented by, for example, a computer 2a. The processing unit 2a1 may be configured to read a program from the storage unit 2a2 and execute the read program to perform various control operations. The program may be stored in the storage unit 2a2 in advance or may be acquired via a medium when needed. The acquired program is stored in the storage unit 2a2 and read from the storage unit 2a2 by the processing unit 2a1 for execution. The medium may be various storage media readable by the computer 2a or a communication line connected to the communication interface 2a3. The processing unit 2a1 may be a CPU (Central Processing Unit). The storage unit 2a2 may include a random access memory (RAM), a read only memory (ROM), a hard disk drive (HDD), a solid state drive (SSD), or a combination thereof. The communication interface 2a3 may communicate with the plasma processing apparatus 1 via a communication line such as a local area network (LAN).

[0045] 2 is a flowchart showing a plasma processing method (hereinafter also referred to as "this processing method") according to one example embodiment. As shown in FIG. 2, this processing method includes a substrate providing step ST1, a first etching step ST2, and a second etching step ST3. The processing in each step may be performed in the plasma processing system shown in FIG. 1. In the following, an example will be described in which a controller 2 controls each part of the plasma processing apparatus 1 to perform this processing method on a substrate W.

[0046] (Process ST1: Providing the substrate) In step ST1, a substrate W is provided in a plasma processing space 10s of the plasma processing apparatus 1. The substrate W is provided in a central region 111a of a substrate support 11. The substrate W is then held on the substrate support 11 by an electrostatic chuck 1111.

[0047] 3 is a diagram showing an example of the cross-sectional structure of the substrate W provided in step ST1. The substrate W has an etching film EF and a mask film MF stacked in this order on an undercoat film UF. The substrate W may be used in the manufacture of semiconductor devices. Examples of semiconductor devices include semiconductor memory devices such as DRAMs and 3D-NAND flash memories.

[0048] The base film UF may be, for example, a silicon wafer, an organic film formed on a silicon wafer, a dielectric film, a metal film, a semiconductor film, etc. The base film UF may be configured by laminating a plurality of films.

[0049] The etching film EF is a film to be etched in this processing method. In one example, the etching film EF is a silicon-containing film. In one example, the silicon-containing film is a silicon oxide film, a silicon nitride film, or the like. The etching film EF may also be a multilayer film consisting of two or more layers.

[0050] The mask film MF is a film that functions as a mask in etching the etching film EF. In one example, the mask film MF may be a carbon-containing film or a silicon-containing film. The silicon-containing film may be, for example, a silicon film, or may further be a film containing a metal or a non-metal. The metal may include tungsten. The non-metal may include boron. The mask film MF may be a single-layer mask consisting of one layer, or a multi-layer mask consisting of two or more layers. The mask film MF may also include an organic film. In one example, the organic film is a photoresist film.

[0051] As shown in FIG. 3, the mask film MF has at least one opening OP. The opening OP is provided in region RE1. The opening OP is a space on the etching film EF surrounded by the side surface ss of the mask film MF. That is, in FIG. 3, a part of the upper surface of the etching film EF is exposed at the opening OP. That is, the etching film EF is exposed in region RE1. Furthermore, the etching film EF is covered by the mask film MF in region RE2. The upper surface of the etching film EF is the surface where the etching film EF contacts the mask film MF. The opening OP may have any shape in a plan view of the substrate W (when the substrate W is viewed from top to bottom in FIG. 3). The shape may be, for example, a circle, an ellipse, a rectangle, or a line. The shape may also be a combination of one or more of these shapes. The mask film MF may have multiple openings OP. In one example, each of the multiple openings OP may have a hole shape. The multiple hole-shaped openings OP may be arranged at regular intervals to form an array pattern. Furthermore, each of the plurality of openings OP may have a linear shape, and the plurality of linear openings OP may be arranged at regular intervals to form a line and space pattern.

[0052] 3 is an example of a first region. Region RE2 is an example of a second region. Region RE1 is a region where the etching film EF is exposed in a plan view of the substrate W. Region RE2 is a region where the mask film MF is exposed in a plan view of the substrate W. That is, region RE2 is a region where the mask film MF covers the etching film EF in a plan view of the substrate W.

[0053] Each of the films constituting the substrate W (base film UF, etching film EF, mask film MF) may be formed by a CVD (Chemical Vapor Deposition) method, an ALD (Atomic Layer Deposition) method, a spin coating method, or the like. Each of the above films may be flat or may have an uneven surface. The opening OP may be formed by etching the mask film MF. The substrate W may further have another film below the base film UF, and the stacked film of the etching film EF and base film UF may function as a multilayer mask. In other words, the stacked film of the etching film EF and base film UF may be used as a multilayer mask to etch the other film.

[0054] At least a part of the process of forming each film on the substrate W may be performed within the space of the plasma processing chamber 10. In one example, the step of etching the mask film MF to form the opening OP may be performed in the plasma processing chamber 10. That is, the opening OP and the etching of the etching film EF, which will be described later, may be performed consecutively within the same chamber. Alternatively, after all or part of each film on the substrate W is formed in an apparatus or chamber external to the plasma processing apparatus 1, the substrate W may be loaded into the plasma processing space 10s and placed in the central region 111a of the substrate support 11, thereby providing the substrate.

[0055] (Step ST2: First Etching) In step ST2, the etching film EF is etched. The etching film EF is etched by plasma generated from a processing gas supplied to the plasma processing chamber 10. That is, first, a processing gas is supplied from the gas supply unit 20 into the plasma processing space 10s. Then, a source RF signal is supplied to the lower electrode of the substrate support unit 11. This generates a high-frequency electric field between the shower head 13 and the substrate support unit 11, and plasma is generated from the processing gas in the plasma processing space 10s. Furthermore, a bias RF signal (bias signal) is supplied to the lower electrode (bias electrode) of the substrate support unit 11, and a bias potential (bias potential) is generated between the plasma and the substrate W. The bias potential attracts active species such as ions and radicals in the plasma to the substrate W, and the active species etch the etching film EF.

[0056] This processing method has first to fourth periods as shown in FIGS. 6A to 7C, which will be described later. Step ST2 includes a first period and a second period as shown in FIGS. 6A to 7C. In both the first period and the second period, the etching film EF is etched. In the first period and the second period, the etching film EF is etched under different etching conditions.

[0057] FIG. 4A is a diagram showing an example of a cross-sectional structure of the substrate W after processing in the first period of step ST2. In step ST2, a portion of the etching film EF exposed at the opening OP is etched in the depth direction (from top to bottom in FIG. 4A ) to form a recess RC. The recess RC is a space surrounded by the side walls of the etching film EF. The side walls are continuous with the side surfaces ss of the mask film MF. A protective film PF is formed on the upper surface ts of the mask film, the side surfaces ss of the mask film, and part of the recess RC during etching. The protective film PF can protect the mask film MF during etching of the etching film EF. The protective film PF includes a protective film PF1 formed on the upper surface ts of the mask film and a protective film PF2 formed on the side surfaces ss of the mask film. The protective film PF2 may be formed from the side surfaces ss to at least part of the recess RC. The protective film PF may be a deposit generated from a carbon-containing gas contained in the processing gas. The protective film PF may be a deposit containing by-products produced by etching the etching film EF.

[0058] The processing gas contains a carbon-containing gas. The carbon-containing gas contributes at least to the deposition of the protective film PF. That is, the protective film PF may be formed by depositing a product generated from the carbon-containing gas by plasma. When the flow rate of the carbon-containing gas in the processing gas is increased, the deposition rate of the protective film PF may be greater than the removal rate of the protective film PF by the oxygen-containing gas described below. The carbon-containing gas contains C a F b (a and b are integers of 1 or more) Gas, or C c H d F e (c, d, and e are integers of 1 or more) are represented by gas, for example, CF4 gas, C2F6 gas, C2F4 gas, C3F8 gas, C4F8 gas, CH2F2 gas, C3H2F4 gas, C3H2F6 gas, C3H3F5 gas, C4H2F6 gas, C4H5F5 gas, C4H2F8 gas, C5H2F6 gas, C5H2F 10 At least one selected from the group consisting of a C5H3F7 gas and a C5H3F7 gas may be used.

[0059] The processing gas may also include an etching gas. The etching gas mainly contributes to etching the etching film EF. The etching gas may contain fluorine, for example, at least one selected from the group consisting of HF gas, NF3 gas, and WF6 gas. The carbon-containing gas may also contribute to etching the etching film EF. That is, the etching film EF may be etched by activated species generated from the carbon-containing gas by plasma. As an example, the activated species include activated species of fluorine.

[0060] The processing gas may also contain an oxygen-containing gas. The oxygen-containing gas may contribute to the removal of the protective film PF deposited during etching. When the flow rate of the oxygen-containing gas in the processing gas increases, the removal rate of the protective film PF may be greater than the deposition rate of the protective film PF. The oxygen-containing gas is a gas containing oxygen, and may be, for example, at least one gas selected from the group consisting of O2 gas, CO gas, and CO2 gas. The processing gas may also contain an inert gas such as Ar.

[0061] 4B is a diagram showing an example of a cross-sectional structure of the substrate W after processing in the second period of step ST2. Similar to the processing in the first period of step ST2, plasma is generated from the processing gas in the plasma processing space 10s, and the etching film EF is etched. This causes the etching film EF to be further etched. That is, the recess RC of the etching film EF in FIG. 4B is deeper than the recess RC in FIG. 4A.

[0062] During the first and second periods, as etching of the etching film EF progresses, the formation of the protective film PF progresses. That is, during the first and second periods, both the formation of the protective film PF and the removal of the protective film PF can occur. During the first and second periods, the flow rates of the carbon-containing gas and the oxygen-containing gas are controlled so that the deposition rate of the protective film PF exceeds the removal rate of the protective film PF. Furthermore, the deposition rate of the protective film PF during the second period is lower than the deposition rate of the protective film PF during the first period. In one example, the flow rate of the carbon-containing gas during the second period is set lower than the flow rate of the carbon-containing gas during the first period, so that the deposition rate of the protective film PF during the second period is lower than the deposition rate of the protective film PF during the first period. Furthermore, the removal rate of the protective film PF during the second period is lower than the removal rate of the protective film PF during the first period. In one example, the flow rate of the oxygen-containing gas in the second period is set to be less than the flow rate of the oxygen-containing gas in the first period, so that the removal rate of the protective film PF in the second period is set to be less than the removal rate of the protective film PF in the first period.

[0063] (Step ST3: Second Etching) In step ST3, the etching film EF is further etched by plasma generated from the processing gas supplied to the plasma processing chamber 10, similar to step ST2.

[0064] FIG. 5A shows an example of the cross-sectional structure of the substrate W after processing in the third period of step ST3. Similar to the process in step ST3, the processing gas contains a carbon-containing gas, an etching gas, and an oxygen-containing gas. Plasma is generated from the processing gas supplied into the plasma processing space 10s, and the etching film EF is etched. The protective film PF1 formed on the upper surface ts of the mask film MF and the protective film PF2 formed on the side surface ss of the mask film and on part of the side wall of the etching film EF at the recess RC are removed as the etching film EF is etched. Note that the recess RC of the etching film EF in FIG. 5A is deeper than the recess RC in FIG. 4B.

[0065] FIG. 5B shows an example of the cross-sectional structure of the substrate W during processing in the fourth period of step ST3. Similar to the process in step ST3, the processing gas contains a carbon-containing gas, an etching gas, and an oxygen-containing gas. Plasma is generated from the processing gas supplied into the plasma processing space 10s, and the etching film EF is etched. The protective film PF1 formed on the upper surface ts of the mask film MF and the protective film PF2 formed on the side surface ss of the mask film and on part of the side wall of the etching film EF at the recess RC are removed as the etching film EF is etched. During the fourth period, the protective film PF may be partially or completely removed. The recess RC of the etching film EF in FIG. 5B is deeper than the recess RC in FIG. 5A. The recess RC may be deep enough to reach the base film UF.

[0066] In the third and fourth periods, as the etching of the etching film EF progresses, the removal of the protective film PF progresses. That is, in the third and fourth periods, both the formation of the protective film PF and the removal of the protective film PF can occur. In the third and fourth periods, the flow rates of the carbon-containing gas and the oxygen-containing gas are controlled so that the removal rate of the protective film PF exceeds the deposition rate of the protective film PF. Furthermore, the removal rate of the protective film PF in the fourth period is greater than the removal rate of the protective film PF in the third period. In one example, the flow rate of the oxygen-containing gas in the fourth period is set greater than the flow rate of the oxygen-containing gas in the third period to control necking in the fourth period. Furthermore, the deposition rate of the protective film PF in the fourth period is greater than the deposition rate of the protective film PF in the third period. In one example, the flow rate of the carbon-containing gas in the fourth period is made higher than the flow rate of the carbon-containing gas in the third period, and the deposition rate of the protective film PF in the fourth period is made higher than the deposition rate of the protective film PF in the third period.

[0067] Steps ST2 and ST3 may be repeatedly performed. The number of times steps ST2 and ST3 are repeated may be set arbitrarily. For example, steps ST2 and ST3 are considered to be one cycle, and it is determined whether the number of cycles has reached a preset number of repetitions, and steps ST2 and ST3 may be repeated until the number of cycles has reached the preset number. The number of repetitions may be set based on the film thickness of the etching film EF (i.e., the depth of the recess RC to be formed). The first cycle may start from a period other than the first period.

[0068] 6A, 6B, and 6C are timing charts showing examples of the flow rates of the carbon-containing gas, oxygen-containing gas, and bias voltage supplied in steps ST2 and ST3, respectively. In FIGS. 6A, 6B, and 6C, the horizontal axis represents time, and the vertical axis represents the flow rate of the carbon-containing gas, the flow rate of the oxygen-containing gas, and bias voltage, respectively.

[0069] As shown in FIG. 6A, in this example, the flow rate of the carbon-containing gas in the first period and the second period is greater than the flow rate of the carbon-containing gas in the third period and the fourth period. The flow rate of the carbon-containing gas in the first period is greater than the flow rate of the carbon-containing gas in the second period. The flow rate of the carbon-containing gas in the third period is less than the flow rate of the carbon-containing gas in the fourth period. The flow rate of the carbon-containing gas in the fourth period is less than the flow rate of the carbon-containing gas in the second period. Thus, in this processing method, the flow rate of the carbon-containing gas in the second period is less than the flow rate of the carbon-containing gas in the first period, so that deposition of the protective film PF can be suppressed in the second period. This allows necking of the protective film PF at the opening OP to be controlled, thereby suppressing or dispersing ions obliquely incident on the opening OP. The flow rate of the carbon-containing gas in the fourth period is greater than the flow rate of the carbon-containing gas in the third period. As a result, particularly when the carbon-containing gas contains an element that etches the etching film EF, a large number of ions of the element can be supplied to the etching film EF with reduced necking during the fourth period, thereby improving the etching rate of the etching film EF.

[0070] 6B, in this example, the flow rate of the oxygen-containing gas in the first period is greater than the flow rate of the oxygen-containing gas in the second period. The flow rates of the oxygen-containing gas in the third and fourth periods are greater than the flow rates of the oxygen-containing gas in the first and second periods. The flow rate of the oxygen-containing gas in the third period is greater than the flow rate of the oxygen-containing gas in the second period. The flow rate of the oxygen-containing gas in the fourth period is greater than the flow rate of the oxygen-containing gas in the third period. The flow rate of the oxygen-containing gas in the third period is greater than the flow rate of the oxygen-containing gas in the first period. In this way, in this processing method, the deposition and removal of the protective film PF can be controlled by controlling the flow rate of the oxygen-containing gas in each period. This makes it possible to control necking of the protective film PF at the opening OP.

[0071] Also, as shown in FIG. 6C , in this example, the bias voltage is constant from the first period to the fourth period. In this example, a bias DC signal is supplied to the substrate support 11 as an example of a bias signal. The bias voltage shown in FIG. 6C has negative polarity. The bias voltage shown in FIG. 6C is an example of bias power. The absolute value of the bias voltage is an example of the effective value of the power of the bias signal. The bias signal may be a bias RF signal. In this case, the effective value of the power of the bias RF signal is an example of the effective value of the power of the bias signal. By keeping the bias voltage constant and gradually increasing or decreasing the flow rates of the carbon-containing gas and the oxygen-containing gas, respectively, it is possible to etch the etching film EF while protecting the mask film MF with the protective film PF. This makes it possible to improve the selectivity to the mask film MF.

[0072] 7A, 7B, and 7C are timing charts showing other examples of the flow rates of the carbon-containing gas, the oxygen-containing gas, and the bias voltage in steps ST2 and ST3, respectively. In FIGS. 7A, 7B, and 7C, the horizontal axis represents time, and the vertical axis represents the absolute values of the flow rates of the carbon-containing gas, the oxygen-containing gas, and the bias voltage.

[0073] 7A, in this example, the flow rate of the carbon-containing gas in the first period and the second period is higher than the flow rate of the carbon-containing gas in the third period and the fourth period. The flow rate of the carbon-containing gas in the first period is higher than the flow rate of the carbon-containing gas in the second period. The flow rate of the carbon-containing gas in the third period is lower than the flow rate of the carbon-containing gas in the fourth period. The flow rate of the carbon-containing gas in the fourth period is lower than the flow rate of the carbon-containing gas in the second period.

[0074] 7B, in this example, the flow rate of the oxygen-containing gas in the first period is greater than the flow rate of the oxygen-containing gas in the second period. The flow rates of the oxygen-containing gas in the third and fourth periods are greater than the flow rates of the oxygen-containing gas in the first and second periods. The flow rate of the oxygen-containing gas in the third period is greater than the flow rate of the oxygen-containing gas in the second period. The flow rate of the oxygen-containing gas in the fourth period is greater than the flow rate of the oxygen-containing gas in the third period. The flow rate of the oxygen-containing gas in the third period is greater than the flow rate of the oxygen-containing gas in the first period.

[0075] As shown in FIG. 7C , in this example, the absolute value of the bias voltage changes stepwise from the first period to the fourth period. In this example, a bias DC signal is supplied to the substrate support 11 as an example of a bias signal. The bias voltage shown in FIG. 7C may have negative polarity. The bias voltage shown in FIG. 7C is an example of bias power. The absolute value of the bias voltage is an example of the effective value of the power of the bias signal. The bias signal may be a bias RF signal. In this case, the effective value of the power of the bias RF signal is an example of the effective value of the power of the bias signal. In this example, the absolute values of the bias voltage in the first and second periods are greater than the absolute values of the bias voltage in the third and fourth periods. The absolute value of the bias voltage in the first period is greater than the absolute value of the bias voltage in the second period. The absolute value of the bias voltage in the third period is smaller than the absolute value of the bias voltage in the fourth period. The absolute value of the bias voltage in the fourth period is the same magnitude as the absolute value of the bias voltage in the first period. The absolute value of the bias voltage in the fourth period may be greater or smaller than that in the first period. By gradually increasing or decreasing the bias voltage in the same manner as the flow rate of the carbon-containing gas, the selectivity of the mask film MF to the deposits PF can be improved while enhancing the protective effect of the mask film MF. Furthermore, in this example, the absolute value of the bias voltage in the second period is set smaller than that in the first period, thereby controlling necking of the protective film PF at the opening OP. Controlling the necking of the protective film PF may include, for example, shifting the necking apex in the etching direction or changing the thickness or width of the necking. Furthermore, in this example, the absolute value of the bias voltage in the fourth period is greater than that in the third period. This increases the energy of ions in a state where necking is reduced in the fourth period, thereby improving the etching rate of the etching film EF. Furthermore, because necking is reduced in the fourth period, ions are less likely to be scattered even if the absolute value of the bias voltage is increased.

[0076] The effective power value of the source RF signal may be controlled during the first to fourth periods. For example, the effective power value of the source RF signal during the first period may be higher than the effective power value of the source RF signal during the second period. The effective power value of the source RF signal during the third period may be higher than the effective power value of the source RF signal during the second period. The effective power value of the source RF signal during the fourth period may be higher than the effective power value of the source RF signal during the third period. The effective power value of the source RF signal during the third period may be higher than the effective power value of the source RF signal during the first period. In this manner, in this processing method, the effective power value of the source RF signal during each period is controlled to control the amount of ions generated from the carbon-containing gas, thereby controlling the deposition and removal of the protective film PF. This allows for the control of necking of the protective film PF at the opening OP.

[0077] The number of periods included in each cycle and the length of each period may be determined appropriately depending on the etching conditions. The etching conditions may include, for example, the types of mask film MF and etching film EF, the types of gases included in the process gas, and the effective values of the power of the source RF signal and bias signal. The gas flow rate, the effective values of the power of the source RF signal and bias signal, the length of each period, and the like may be determined appropriately depending on the target and purpose of etching.

[0078] During the first to fourth periods, the flow rate of the carbon-containing gas may be changed stepwise from the flow rate in one period to the flow rate in another period when transitioning from one period to another. Alternatively, during the first to fourth periods, the flow rate of the carbon-containing gas may be changed continuously from the flow rate in one period to the flow rate in another period when transitioning from one period to another. That is, the flow rate of the carbon-containing gas may be changed immediately at t0, t1, t2, t3, and / or t4, or may be changed gradually in steps or continuously.

[0079] When transitioning from one period to another during the first to fourth periods, the flow rate of the oxygen-containing gas may be changed stepwise from the flow rate in one period to the flow rate in another period. Alternatively, when transitioning from one period to another during the first to fourth periods, the flow rate of the oxygen-containing gas may be changed continuously from the flow rate in one period to the flow rate in another period. That is, the flow rate of the oxygen-containing gas may be changed immediately at t0, t1, t2, t3, and / or t4, or may be changed gradually in steps or continuously.

[0080] When transitioning from one period to another during the first to fourth periods, the bias voltage may change stepwise from the voltage in one period to the voltage in another period. Alternatively, when transitioning from one period to another during the first to fourth periods, the bias voltage may change continuously from the voltage in one period to the voltage in another period. That is, the bias voltage may change immediately at t0, t1, t2, t3, and / or t4, or may change gradually in steps or continuously.

[0081] FIG. 8 is a diagram showing an example of a cross-sectional structure of a substrate W in another example of the present processing method. In this example, the substrate W has a silicon nitride film NF and a silicon oxide film OF provided adjacent to each other in the etching direction of the silicon oxide film OF (i.e., the lateral direction in FIG. 8) on an undercoat film UF. The silicon nitride film NF is an example of a mask film. The silicon oxide film OF is also an example of an etching film. In this example, as in the examples described with reference to FIGS. 3 to 7, the silicon oxide film OF can be etched while a protective film PF is formed on the silicon nitride film NF. This makes it possible to improve the selectivity of the silicon oxide film OF relative to the silicon nitride film NF. Note that another mask film may be further formed on the silicon nitride film NF.

[0082] 8 is an example of a first region. Region RE2 is an example of a second region. Region RE1 is a region where a silicon oxide film OF is provided on an underlayer UF in a plan view of the substrate W. Region RE2 is a region where a silicon nitride film NF is provided on an underlayer UF in a plan view of the substrate W.

[0083] This processing method can control the position and shape of the necking of the mask film MF and the protective film PF. This can improve the etching selectivity between the mask film MF and the protective film PF and the etching film EF while suppressing necking. For example, this processing method shifts the position of the necking apex during etching, dispersing the positions where ions obliquely incident on the opening OP collide with the sidewall of the recess RC. This can suppress bowing of the recess RC formed in the etching film EF. Furthermore, as the necking shifts downward, etching of the etching film EF can progress further. As the necking shifts downward, the opening OP is unblocked, making it easier for ions in the plasma to enter, thereby increasing the etching rate of the etching film EF.

[0084] Furthermore, this processing method can improve the selectivity with respect to the etching film EF. The selectivity is the ratio of the etching rate of the etching film EF to the etching rate of the mask film MF. In particular, the deeper the recess RC, the higher the selectivity can be. Furthermore, the verticality of the recess RC can also be improved.

[0085] The above embodiments have been described for illustrative purposes, and various modifications may be made without departing from the scope and spirit of the present disclosure. For example, the present processing method may be performed using a plasma processing apparatus using any plasma source, such as an inductively coupled plasma or microwave plasma, in addition to the capacitively coupled plasma processing apparatus 1. [Explanation of symbols]

[0086] 1: Plasma processing apparatus, 2: Control unit, 10: Plasma processing chamber, 10s: Plasma processing space, 11: Substrate support unit, 20: Gas supply unit, PF: Protective film, MF: Mask film, EF: Etching film, UF: Undercoat film, W: Substrate, OP: Opening, RC: Recess, ts: Upper surface of mask film, ss: Side surface of mask film

Claims

1. A plasma processing method performed in a plasma processing apparatus having a chamber, comprising: (a) providing a substrate having an etching film and a masking film in the chamber, the substrate having a first region where the etching film is exposed and a second region where the masking film is exposed; (b) supplying a process gas containing a carbon-containing gas into the chamber to generate plasma from the process gas to etch the etching film and form a protective film on the mask film; (c) supplying the processing gas into the chamber to generate plasma from the processing gas to further etch the etching film and remove at least a portion of the protective film; the step (b) includes a first period and a second period, and the flow rate of the carbon-containing gas in the first period is greater than the flow rate of the carbon-containing gas in the second period; the step (c) includes a third period and a fourth period, and the flow rate of the carbon-containing gas in the third period is lower than the flow rate of the carbon-containing gas in the second period and the flow rate of the carbon-containing gas in the fourth period; Plasma treatment method.

2. the mask film is provided on the etching film, the mask film has an upper surface that covers the etching film in the second region and a side surface that defines an opening through which the etching film is exposed in the first region; the step (b) includes etching the etching film to form a recess in the etching film in the first region, and forming the protection film on at least the upper surface of the mask film; The plasma processing method of claim 1 , wherein the step (c) includes removing at least a portion of the protective film.

3. 3. The plasma processing method of claim 1, wherein a flow rate of the carbon-containing gas in the fourth period is lower than a flow rate of the carbon-containing gas in the second period.

4. 4. The plasma processing method according to claim 1, wherein the flow rate of the carbon-containing gas changes continuously or stepwise when transitioning from one period to another period among the first period to the fourth period.

5. the plasma processing apparatus further includes a substrate support within the chamber; (a) includes providing the substrate on the substrate support; The plasma processing method of claim 1 , wherein (b) and (c) comprise applying a bias signal to the substrate support.

6. 6. The plasma processing method according to claim 5, wherein in (b), the effective value of the power of the bias signal in the first period is greater than the effective value of the power of the bias signal in the second period.

7. 7. The plasma processing method according to claim 5, wherein in (c), the effective value of the power of the bias signal in the third period is smaller than the effective value of the power of the bias signal in the fourth period.

8. 8. The plasma processing method according to claim 6, wherein the effective value of the power of the bias signal changes continuously or stepwise when transitioning from one period to another of the first period to the fourth period.

9. 9. The plasma processing method of claim 1, wherein the step (b) includes supplying a source RF signal to the chamber to generate plasma, and wherein an effective value of power of the source RF signal in the first time period is greater than an effective value of power of the source RF signal in the second time period.

10. 10. The plasma processing method of claim 9, wherein (c) includes supplying a source RF signal to the chamber to generate plasma, and wherein an effective value of power of the source RF signal in the third time period is smaller than an effective value of power of the source RF signal in the fourth time period.

11. 11. The plasma processing method according to claim 1, wherein the processing gas contains an oxygen-containing gas, and in (b), a flow rate of the oxygen-containing gas in the first period is higher than a flow rate of the oxygen-containing gas in the second period.

12. 12. The plasma processing method according to claim 1, wherein the processing gas contains an oxygen-containing gas, and in (c), a flow rate of the oxygen-containing gas in the third period is lower than a flow rate of the oxygen-containing gas in the fourth period.

13. the substrate further comprises an undercoat film; the etching film is provided in the first region, 13. The plasma processing method according to claim 1, wherein the mask film is provided in the second region so as to be adjacent to the etching film in a direction perpendicular to a direction in which the etching film is etched.

14. 14. The plasma processing method according to claim 1, wherein the etching film is a silicon oxide film or a silicon nitride film.

15. 15. The plasma processing method according to claim 1, wherein the mask film includes at least one of a boron-containing silicon film, a carbon-containing film, a nitrogen-containing film, and a tungsten silicon film.

16. The carbon-containing gas is C a F b (a and b are integers of 1 or more) gas or C c H d F e 16. The plasma processing method according to claim 1, wherein c, d, and e are integers of 1 or more.

17. The plasma processing method according to claim 1 , wherein the steps (b) and (c) are repeated multiple times.

18. 3. The plasma processing method according to claim 2, wherein, during at least a part of the first period to the fourth period, a top of the protective film formed on the side surface of the mask film shifts in a direction in which the etching film is etched as etching of the etching film progresses.

19. A plasma processing method performed in a plasma processing apparatus having a chamber, comprising: (a) providing a substrate in a chamber, the substrate having a first region including an exposed portion of the etching film and a second region including an exposed portion of the mask film; (b) a first step of supplying a process gas containing a carbon-containing gas at a first flow rate into the chamber to generate plasma, thereby etching the etching film and forming a protective film on the mask film; (c) a second step of supplying a process gas containing a carbon-containing gas at a second flow rate less than the first flow rate into the chamber to generate plasma, thereby further etching the etching film and forming the protective film on the mask film; (d) a third step of supplying a process gas containing a carbon-containing gas at a third flow rate into the chamber to generate plasma, thereby further etching the etching film and removing the protective film on the mask film; (e) a fourth step of supplying a carbon-containing gas into the chamber at a fourth flow rate that is less than the second flow rate and greater than the third flow rate to generate plasma, thereby further etching the etching film and further removing the protective film on the mask film.

20. 1. A plasma processing system having a chamber, a process gas supply, a plasma generation unit, and a control unit, The control unit (a) providing a substrate having an etching film and a mask film in the chamber; (b) the processing gas supply unit supplies a processing gas containing a carbon-containing gas into the chamber, and the plasma generation unit generates plasma from the processing gas to etch the etching film and form a protective film on the mask film; (c) the processing gas supply unit supplies the processing gas into the chamber, and the plasma generation unit generates plasma from the processing gas to further etch the etching film and remove at least a portion of the protective film; Execute control, the substrate has a first region where the etching film is exposed and a second region where the mask film is exposed; the step (b) includes a first period and a second period, and the flow rate of the carbon-containing gas in the first period is greater than the flow rate of the carbon-containing gas in the second period; the step (c) includes a third period and a fourth period, and the flow rate of the carbon-containing gas in the third period is less than the flow rate of the carbon-containing gas in the second period and the flow rate of the carbon-containing gas in the fourth period.

Images