Patent Publication Number: US-2020303169-A1

Title: Plasma processing method and plasma processing apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is based on and claims priority to Japanese Patent Application No. 2019-053837 filed on Mar. 20, 2019, the entire contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to a plasma processing method and a plasma processing apparatus. 
     BACKGROUND 
     There is known a dry etching device that detects an endpoint of etching based on a change in emission intensity of the plasma. For example, Patent Document 1 describes a dry etching device including a first detector for measuring emission intensity of a plasma in the vicinity of a semiconductor substrate and a second detector for measuring emission intensity of a plasma in the vicinity of a material to be etched that is placed on a first electrode. Patent Document 1 proposes that the emission intensity measured by the first detector and the emission intensity measured by the second detector are calculated, and that an endpoint of etching is detected by a change in the calculation result. 
     CITATION LIST 
     Patent Document 
     [Patent Document 1] Japanese Laid-open Patent Application Publication No. 09-055367 
     SUMMARY 
     In optical emission spectroscopy (OES), a state of a plasma can be detected by measuring emission intensity of the plasma. A plasma is affected by a state of an inner wall of a plasma processing apparatus. Therefore, it is important to control the state of the inner wall of the plasma processing apparatus. 
     The present disclosure provides a plasma processing method and a plasma processing apparatus capable of accurately determining a state of an inner wall of a plasma processing apparatus. 
     According to one aspect of the present disclosure, there is provision of a plasma processing method using a detector configured to measure emission intensity of a plasma in a plasma processing apparatus. The method includes detecting a first plasma emission intensity that is an intensity of first light incident on the detector through a plasma generating region; detecting at least one second plasma emission intensity that is an intensity of second light incident on the detector from an inner wall of the plasma processing apparatus, without passing through the plasma generating region; and determining a state of the inner wall of the plasma processing apparatus based on a difference between the first plasma emission intensity and the second plasma emission intensity, or based on a ratio between the first plasma emission intensity and the second plasma emission intensity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are cross-sectional diagrams each illustrating a plasma processing apparatus according to an embodiment; 
         FIG. 2  is a diagram illustrating an example of a detector according to the embodiment; 
         FIG. 3  is a view illustrating an example of a detection result according to the embodiment; 
         FIGS. 4A and 4B  are diagrams illustrating differences (wall condition) in emission intensity according to the embodiment; 
         FIG. 5  is a flowchart illustrating a measurement process of plasma emission intensity according to the embodiment; 
         FIG. 6  is a flowchart illustrating a seasoning process according to the embodiment; 
         FIG. 7  is a view illustrating how to determine initiation and termination of conditioning according to the embodiment; 
         FIG. 8  is a flowchart illustrating a dry cleaning process according to the embodiment; 
         FIG. 9  is a flowchart illustrating a dry cleaning process according to a first variation of the embodiment; 
         FIG. 10  is a diagram illustrating an example of a detector according to the first variation of the embodiment; and 
         FIG. 11  is a flowchart illustrating a seasoning process according to a second variation of the embodiment. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments for carrying out the present disclosure will be described with reference to the drawings. In each drawing, the same components are indicated by the same reference numerals and overlapping descriptions may be omitted. 
     [Plasma Processing Apparatus] 
     First, a plasma processing apparatus  10  that performs a plasma processing method according to an embodiment will be described with reference to  FIG. 1 .  FIG. 1A  and  FIG. 1B  illustrate cross-sectional schematic views of a parallel plate type capacitive coupling (CCP) plasma processing apparatus, as an example of a plasma processing apparatus  10  according to the present embodiment. 
     First, a configuration of the plasma processing apparatus  10  illustrated in  FIG. 1A  will be described. The plasma processing apparatus  10  includes a processing vessel  11  and a stage  12  disposed therein. The processing vessel  11  is a cylindrical container. The processing vessel  11  is, for example, made of aluminum with an alumite-treated (anodized) surface, and is grounded. The stage  12  has a base  16  and an electrostatic chuck  13  disposed on the base  16 . The stage  12  is disposed at the bottom of the processing vessel  11  via a support  14  formed of an insulating member. 
     The base  16  is formed of aluminum or the like. The electrostatic chuck  13  is formed of a dielectric material such as alumina (Al 2 O 3 ), and has a mechanism for holding a wafer W with electrostatic attracting force. In the electrostatic chuck  13 , a wafer W is placed in the center, and an annular edge ring  15  (also referred to as a focus ring) surrounding the wafer W is placed in the outer circumference. 
     An annular exhaust path  23  is formed between a side wall of the processing vessel  11  and a side wall of the stage  12 , and the exhaust path  23  is connected to the exhaust device  22  via an exhaust port  24 . The exhaust device  22  includes a vacuum pump such as a turbomolecular pump or a dry pump. The exhaust device  22  directs a gas in the processing vessel  11  to the exhaust path  23  and the exhaust port  24 , and evacuates the gas. This reduces pressure of a processing space in the processing vessel  11  to a predetermined quality of vacuum. 
     The exhaust path  23  is provided with a baffle plate  27  that separates the processing space from an exhaust space and that controls a flow of gas. The baffle plate  27  is an annular member coated with, for example, a corrosion-resistant film (e.g., yttrium oxide (Y 2 O 3 )) on a surface of a base material formed of aluminum, and multiple through-holes are formed in the baffle plate  27 . 
     The stage  12  is connected to a first radio frequency power supply  17  and a second radio frequency power supply  18 . The first radio frequency power supply  17  applies, for example, 60 MHz of radio frequency electric power for plasma generation (hereinafter referred to as “HF power”) to the stage  12 . The second radio frequency power supply  18  applies, for example, 40 MHz of radio frequency electric power for attracting ions (hereinafter referred to as “LF power”) to the stage  12 . Thus, the stage  12  also functions as a lower electrode. 
     At an opening of a ceiling of the processing vessel  11 , a showerhead  20  is provided via a ring-shaped insulating member  28  attached to a circumference of the showerhead  20 . HF power is applied capacitively between the stage  12  and the showerhead  20 , and a gas is formed into a plasma by the HF power primarily. 
     The ions in the plasma are drawn into the stage  12  by LF power applied to the stage  12 , and a wafer W placed on the stage  12  is bombarded with the ions. Accordingly, a predetermined film on the wafer W is efficiently processed (e.g., etched). 
     The gas source  19  supplies a gas according to a process condition of each plasma process, such as an etching process, a cleaning process, and a seasoning process. The gas enters the showerhead  20  via a gas line  21 , passes through a gas diffusion chamber  25 , and is introduced into the processing vessel  11  in a form of a shower from a large number of gas holes  26 . 
     The plasma processing apparatus  10  illustrated in  FIG. 1B  has substantially the same configuration as the plasma processing apparatus  10  illustrated in  FIG. 1A , but arrangement of the first radio frequency power supply  17  and a configuration of a detector  40  to be described below are different. In the plasma processing apparatus  10  illustrated in  FIG. 1B , the first radio frequency power supply  17  is connected to the showerhead  20 . The first radio frequency power supply  17  applies, for example, 60 MHz of HF power to the showerhead  20 . 
     The controller  30  includes a CPU (Central Processing Unit), a ROM (Read Only Memory), and a RAM (Random Access Memory). By the CPU executing a program stored in the RAM or the ROM, the controller  30  controls various components in the plasma processing apparatus  10 . The controller  30  controls various plasma processing and an entire apparatus, in accordance with procedures set in a recipe stored in the RAM. 
     When performing plasma processing in the plasma processing apparatus  10  of the above-described configuration, a wafer W is first loaded into the processing vessel  11  from a gate valve (not illustrated), while the wafer W is held on a transport arm. The wafer W is placed on the electrostatic chuck  13 . The gate valve is closed after the wafer W is loaded. By applying DC voltage to an electrode (not illustrated) of the electrostatic chuck  13 , the wafer W is attracted and held to the electrostatic chuck  13  by Coulomb force. 
     Pressure in the processing vessel  11  is reduced to a set value by the exhaust device  22 , and the interior of the processing vessel  11  is controlled to be in a vacuum state. A predetermined gas is introduced into the processing vessel  11  from the showerhead  20  in a form of a shower. HF power and LF power are applied to the stage  12 . In  FIG. 1A , because the HF power is applied to the stage  12 , a plasma generating region P is in the vicinity of the wafer W. In  FIG. 1B , because the HF power is applied to the showerhead  20 , the plasma generating region P is in the vicinity of the showerhead  20 . 
     A plasma is generated from the introduced gas, mainly by the HF power, and plasma processing such as etching is applied to the wafer W by the plasma. After the plasma processing is completed, the wafer W is held on the transfer arm and is unloaded to the outside of the processing vessel  11 . By repeating this process, wafers W are processed consecutively. 
     Light emitted within the processing vessel  11  during plasma processing of wafer W is incident on the detector  40  via a window  41  provided at the side wall. 
     The detector  40  illustrated in  FIG. 1A  detects light incident along an optical axis of an optical system of the detector  40 . As the detector  40  can change a direction of the optical axis of the optical system, the detector  40  can detect light incident from different directions. In a case in which the optical axis of the optical system of the detector  40  is a dashed line L 1  (hereinafter referred to as a “first optical axis L 1 ”) illustrated in  FIG. 1A , the detector  40  detects light incident from the processing vessel  11  along the first optical axis L 1 . In a case in which the optical axis of the optical system of the detector  40  is a dashed line L 2  (hereinafter referred to as an “optical axis L 2 ”) illustrated in  FIG. 1A , the detector  40  detects light incident from the processing vessel  11  along the optical axis L 2 . Light incident along the first optical axis L 1  passes through the plasma generating region P near the wafer W. After the detector  40  finishes detecting light incident along the first optical axis L 1 , the detector  40  changes a direction of the optical axis from the first optical axis L 1  to the optical axis L 2 , and the detector  40  detects light incident along the second optical axis L 2 . As illustrated in  FIG. 1A , the second optical axis L 2  is directed to a ceiling wall  11   a  of the processing vessel  11  without passing through the plasma generating region P. 
     In  FIG. 1B , the detector  40  includes a detector  40   a  and a detector  40   b . The detectors  40   a  and  40   b  do not have functions to change directions of optical axes of optical systems of the detectors  40   a  and  40   b , respectively. A dashed line L 1  of  FIG. 1B  indicates the optical axis (hereinafter referred to as a first optical axis L 1 ) of the optical system of the detector  40   a , and a dashed line L 2  of  FIG. 1B  indicates the optical axis (hereinafter referred to as a second optical axis L 2 ) of the optical system of the detector  40   b . Light incident along the first optical axis L 1  and light incident along the second optical axis L 2  are emitted from the interior of the processing vessel  11 , and are detected by the detectors  40   a  and  40   b , by passing through different windows  41   a  and  41   b  respectively. However, the plasma processing apparatus  10  illustrated in  FIG. 1B  may be configured such that both the light may pass through the same window, as illustrated in  FIG. 1A . 
     The detector  40   a  detects light incident along the first optical axis L 1 . The first optical axis L 1  passes through the plasma generating region P near the showerhead  20 , and reaches the side wall lib of the processing vessel  11 . The detector  40   b  detects light incident along the second optical axis L 2 . The second optical axis L 2  reaches the side wall  11   b  of the processing vessel  11  without passing through the plasma generating region P. 
     The detector  40  monitors a state of a plasma using OES. In OES, elements in a material are vaporized and excited by a discharge plasma, and wavelengths of emission line spectra (atomic spectra) each of which is unique in each element are qualitatively determined, and intensity of the emission lines is determined quantitatively. However, OES is an example of a technique for monitoring a state of a plasma, and the detector  40  is not limited to OES as long as the detector  40  can monitor a state of a plasma. 
     As illustrated in  FIG. 2 , the detector  40  can change a direction of an optical axis of the detector  40  upward, downward, leftward, rightward, and diagonally by power of an actuator  42 . With such a configuration, the detector  40  can change a direction of light to be detected by the detector  40 , such as a direction corresponding to a line (first optical axis) L 1  and a direction corresponding to a dashed line (second optical axis) L 2 , as illustrated in  FIG. 2 . The detector  40  may also change its optical axis to other directions, such as a second dashed line L 2 ′ in  FIG. 2 . This allows measurement of emission intensity of plasma incident from multiple directions. Also, as will be described later, a state of the inner wall of the processing vessel  11  can be determined based on a difference between plasma emission intensity at multiple locations. In a case in which multiple light beams each of which is incident along a different optical axis are detected through the same window  41 , influence of cloudiness or the like of the window  41  can be cancelled in determining the condition of the inner wall using the difference between the plasma emission intensities at the multiple locations. 
       FIG. 3  illustrates a result of measuring emission spectra of light incident along the line L 1  illustrated in  FIG. 2  and light incident along the dashed line L 2  illustrated in  FIG. 2 , while argon gas is supplied into the processing vessel  11  at a predetermined flow rate. A line labeled as “I L1 ” indicates the emission spectrum of light incident along the line L 1  in  FIG. 2 , and a dashed line labeled as “I L2 ” indicates the emission spectrum of light incident along the dashed line L 2  in  FIG. 2 . Note that the emission spectra in  FIG. 3  are normalized by emission intensity of light that is measured while argon gas is supplied into the processing vessel  11  at another flow rate. According to  FIG. 3 , there was no significant difference between a state of the plasma at the center of the processing vessel  11  measured by the light incident along the line L 1 , and a state of the plasma close to the inner surface of the processing vessel  11  measured by the light incident along the line L 2 . That is, it was found that the measurement results of the state of the plasma at the center of the processing vessel  11  and the measurement results of the state of the plasma at the inner end of the processing vessel  11  were affected by disturbance to the same degree. 
     Therefore, a difference between emission intensity of a predetermined wavelength of light incident along the first optical axis L 1  (hereinafter referred to as “first emission intensity”; each filled circle (IL′) in  FIG. 4A  indicates the first emission intensity) and emission intensity of a predetermined wavelength of light incident along the second optical axis L 2  at the same timing (hereinafter referred to as “second emission intensity”; each white circle (I L2 ) in  FIG. 4A  indicates the second emission intensity) is calculated periodically. In the example of  FIG. 2 , the first optical axis L 1  passes through the plasma generating region P. Thus, the first emission intensity of light incident along the first optical axis L 1  (filled circles (I L1 )) indicates states of a plasma in the plasma generating region P. 
     Meanwhile, the second optical axis L 2  reaches the ceiling wall of the processing vessel  11  without passing through the plasma generating region P. Thus, the second emission intensity of light incident along the second optical axis L 2  (white circles (I L2 )) indicates a state of a plasma diffused from the plasma generating region P and a state of the inner wall of the processing vessel  11 . 
     Thus, by calculating the difference between the first emission intensity and the second emission intensity indicated by arrows in  FIG. 4A , the state of the plasma and the disturbance can be cancelled as factors. That is, the state of the inner wall of the processing vessel  11  can be determined based on the difference between the first and second emission intensity (circles “I L1 -I L2 ” filled with dotted patterns in  FIG. 4B ). 
     [Measurement Process of Plasma Emission Intensity] 
     Hereinafter, a measurement process of plasma emission intensity for determining the state of the wall will be described with reference to  FIG. 5 .  FIG. 5  is a flow chart illustrating the measurement process of plasma emission intensity according to the present embodiment. 
     In the present embodiment, while a plasma is generated in the processing vessel  11 , the detector  40  measures intensity of a predetermined wavelength component in an optical spectrum of plasma light, by optical emission spectroscopy (OES). As described above, intensity of multiple light incident from different directions (or locations), such as light incident along the first optical axis L 1  and light incident along the second optical axis L 2  (in  FIG. 1A or 1B ), are detected by the detector  40 . In the following description, intensity of light incident along the first optical axis L 1  may be referred to as “first plasma emission intensity”, and intensity of light incident along the second optical axis L 2  may be referred to as “second plasma emission intensity”. When the measurement process is initiated, the detector  40  measures the first plasma emission intensity using light incident along the first optical axis L 1  passing through the plasma generating region P, based on OES (step S 1 ). Next, the detector  40  measures the second plasma emission intensity using light incident along the second optical axis L 2  that does not pass through the plasma generating region P, based on OES (step S 2 ). 
     Next, in step S 3 , the controller  30  determines whether or not to terminate the measurement process. If it is determined that the measurement process is to be terminated, the controller  30  terminates the measurement process. If it is determined that the measurement should not be terminated in step S 3 , the controller  30  determines whether or not it is time for the next measurement (step S 4 ). The controller  30  waits until the next measurement time comes. If it is determined that it is the time for the next measurement (YES in step S 4 ), the measurement process returns to step S 1 , and the measurement of the first plasma emission intensity and the next second plasma emission intensity is performed again in steps S 1  and thereafter. Because steps S 1  to S 4  are repeated until the measurement process terminates, the first and second plasma emission intensities are measured at predetermined intervals. The first and second plasma emission intensities measured at the predetermined intervals are transmitted to the controller  30 . 
     [Seasoning Control Process] 
     Next, a seasoning control process using a difference between the first plasma emission intensity and the next second plasma emission intensity will be described with reference to  FIGS. 6 and 7 .  FIG. 6  is a flowchart illustrating the seasoning control process according to the present embodiment.  FIG. 7  is a diagram illustrating the beginning and end of conditioning according to the present embodiment. Hereinafter, as examples of the conditioning, seasoning and dry cleaning will be described. The horizontal axis of  FIG. 7  indicates time, but may indicate the number of wafers (product wafers or dummy wafers). 
     When the seasoning control process (the process of  FIG. 6 ) is started, the controller  30  determines whether or not to start seasoning (step S 10 ).  FIG. 7  describes an example in which the seasoning starts at predetermined time T 0 . Accordingly, the controller  30  determines that the seasoning is started when the time T 0  comes (YES in step S 10 ), and executes the seasoning in the processing vessel (step S 12 ). If seasoning is already being executed, the controller  30  continues the seasoning. During the seasoning, to stabilize the interior of the processing vessel  11 , a plasma is generated under the same process condition as a condition of a wafer process, and a plasma process is performed within the processing vessel  11 . 
     During the seasoning, the first and second plasma emission intensities measured at the detector  40  is sent to the controller  30  at the predetermined intervals. As illustrated in  FIG. 6 , the controller  30  acquires a measurement value of the first plasma emission intensity and a measurement value of the second plasma emission intensity, and calculates a difference between the measurement value of the first plasma emission intensity and the measurement value of the second plasma emission intensity (step S 14 ). 
     Next, in step S 16 , the controller  30  determines whether the calculated difference is within a normal range. Each filled circle in  FIG. 7  indicates magnitude of the difference between the first plasma emission intensity and the second plasma emission intensity (hereinafter simply referred to as a “difference”) and the time when the difference is calculated. Also, in  FIG. 7 , the normal range corresponds to an area between two horizontal dashed lines. Thus, the leftmost filled circle in  FIG. 7  indicates magnitude of the difference calculated for the first time. As illustrated in  FIG. 7 , differences calculated at first and second times during the seasoning are plotted outside the normal range. Meanwhile, differences calculated at third to sixth times during the seasoning are plotted within the normal range. 
     If it is determined the difference calculated in step S 14  is outside the normal range (NO in step S 16 ), the process of  FIG. 6  returns to step S 12  to continue the seasoning. Meanwhile, if it is determined the difference calculated in step S 14  is within the normal range (YES in step S 16 ), the process of  FIG. 6  proceeds to step S 18 . In a case in which the measured differences are as illustrated in  FIG. 7 , with respect to first and second determination in step S 16 , the controller  30  determines that the differences calculated at the first and second times are outside the normal range. Thus, the process of  FIG. 6  returns to step S 12 , and continues the seasoning. Meanwhile, with respect to third to sixth determination in step S 16 , the controller  30  determines that the difference calculated at third to sixth times is within the normal range, and the process of  FIG. 6  proceeds to step S 18 . 
     In step S 18 , the controller  30  determines whether or not a second predetermined period of time has elapsed since the difference became within the normal range for the first time. In a case in which the measured differences are as illustrated in  FIG. 7 , when the difference was calculated for the third intensity measurement (i.e., the third filled circle), the difference became within the normal range for the first time. The second predetermined period of time is a predetermined value in which it is determined that an environment inside the processing vessel  11  has been stabilized in a normal state by performing seasoning. 
     If it is determined that the second predetermined period of time has not elapsed since the difference became within the normal range for the first time (NO in step S 18 ), the controller  30  performs steps S 12  to S 18  again. In a case in which the measured differences are as illustrated in  FIG. 7 , before time T 1 , the controller  30  determines that the second predetermined period of time has not elapsed since the difference became within the normal range for the first time. 
     Meanwhile, if it is determined that the second predetermined period of time has elapsed since the difference became within the normal range for the first time (YES in step S 18 ), the controller  30  terminates seasoning (step S 20 ), and terminates the process of  FIG. 6 . In the example of  FIG. 7 , a determination to terminate seasoning is made at time T 1 . 
     The above-described process determines whether a state of the inner wall of the processing vessel  11  is in the normal range, based on the difference between the first plasma emission intensity and the second plasma emission intensity. Then, based on the determination result, if it is determined that the state of the inner wall of the processing vessel  11  has been stabilized in the normal state, the controller  30  determines that seasoning should be terminated. 
     [Dry Cleaning Control Process] 
     Next, a dry cleaning control process using the difference between the first plasma emission intensity and the second plasma emission intensity will be described with reference to  FIGS. 8 and 7 .  FIG. 8  is a flowchart illustrating the dry cleaning control process according to the present embodiment. 
     When the dry cleaning control process (the process of  FIG. 8 ) is started, the controller  30  controls the transfer arm (not illustrated) to load a wafer W into the processing vessel  11  (step S 30 ). Next, the controller  30  applies HF power and LF power based on process conditions set in the recipe, supplies a predetermined gas to generate a plasma, and applies a plasma process to the wafer W (step S 32 ). 
     Next, after the plasma processing, the controller  30  controls the transfer arm (not illustrated) to unload the wafer W from the processing vessel  11  (step S 34 ). Next, the controller  30  acquires respective measurement values of the first plasma emission intensity and the second plasma emission intensity, and calculates a difference between the first plasma emission intensity and the second plasma emission intensity (step S 36 ). In step S 38 , the controller  30  determines whether the difference between the first plasma emission intensity and the second plasma emission intensity exceeds a first threshold value. The first threshold is a preset value, which is set to a value determined to require dry cleaning due to aggravation of the wall condition in the processing vessel  11 , such as adhesion of deposits on the walls. 
     If it is determined that the difference does not exceed the first threshold value in step S 38 , the controller  30  determines that it is not necessary to start dry cleaning in the processing vessel  11  including the inner wall, and the process of  FIG. 8  returns to step S 30 . The controller  30  loads a next wafer W in the processing vessel  11 , and repeats a process in steps S 32  to S 38 . 
     If it is determined that the difference exceeds the first threshold value in step S 38 , the controller  30  executes dry cleaning (step S 40 ). If the dry cleaning is already being executed, the controller  30  continues the dry cleaning. 
     In the example of  FIG. 7 , the first threshold value is set to an upper limit of the above-described normal range. In this case, processing of wafers W is performed until the difference exceeds the first threshold value. 
     In  FIG. 7 , the difference exceeds the first threshold at the time just before time T 2 , for example. Therefore, it is determined that dry cleaning should be started at this time point. As a result, in the example of  FIG. 7 , dry cleaning has started at time T 2 . 
     After step S 40 , the controller  30  calculates the difference between the first plasma emission intensity and the second plasma emission intensity by performing the same operation as step S 36  (step S 42 ). Next, the controller  30  determines whether the difference calculated in step S 42  is within the normal range (step S 44 ). The controller  30  continues the dry cleaning until the calculated difference becomes within the normal range. If it is determined that the calculated difference is within the normal range in step S 44 , the controller  30  determines whether or not a first predetermined period of time has elapsed since the difference became within the normal range for the first time (step S 46 ). 
     The first predetermined period of time is a predetermined value, in which it is determined that the environment inside the processing vessel  11  has been stabilized in a normal state by dry cleaning. 
     If it is determined that the first predetermined period of time has not elapsed since the difference became within the normal range for the first time (NO in step S 46 ), the controller  30  repeats a process in steps S 40  to S 46 . 
     Meanwhile, if it is determined that the first predetermined period of time has elapsed since the difference became within the normal range (YES in step S 46 ), the controller  30  terminates dry cleaning (step S 48 ). Then, the process of  FIG. 8  returns step S 30 , and the controller  30  repeats step S 30  and thereafter. 
     In the example of  FIG. 7 , dry cleaning is terminated at time T 3 , which is a time when a state in which the difference between the first plasma emission intensity and the second plasma emission intensity is within the normal range has been continued for the first predetermined period of time, after the difference between the first plasma emission intensity and the second plasma emission intensity had exceeded the first threshold value and since the difference had returned to the normal range for the first time. 
     As a result, next wafers W are processed during the time between T 3  and T 4  in  FIG. 7 . When the difference between the first plasma emission intensity and the second plasma emission intensity exceeds the first threshold value again (just before time T 4  in  FIG. 7 ), it is determined that dry cleaning is necessary, and dry cleaning is started at time T 4 . 
     In the example of  FIG. 8 , a wafer W was processed immediately after completion of the dry cleaning, but this is not limited thereto. For example, a predetermined film may be pre-coated for a predetermined period of time after completion of dry cleaning, and processing of wafer W may be performed after the pre-coating. 
     As described above, in the plasma processing according to the present embodiment, the condition of the wall in the processing vessel  11  is determined based on the difference between the first plasma emission intensity indicating a state of a plasma and the second plasma emission intensity indicating a state of the plasma and the wall. This allows termination of seasoning, start of dry cleaning, and termination of dry cleaning to be performed in a timely manner based on the condition of the wall. This can avoid decrease in productivity of wafer processing due to deterioration of the environment in the processing vessel  11 , such as generation of particles in the processing vessel  11 . 
     [First Variation] 
     Next, a dry cleaning control process according to a first variation of the present embodiment will be described with reference to  FIGS. 9 and 10 .  FIG. 9  is a flowchart illustrating the dry cleaning control process according to the first variation of the present embodiment.  FIG. 10  is a diagram illustrating an example of the detector  40  according to the first variation of the present embodiment. Among steps of the dry cleaning control process according to the first variation of  FIG. 9 , the same step numbers are assigned to the steps of performing the same process as the dry cleaning control process of  FIG. 8 . 
     As illustrated in  FIG. 10 , in the dry cleaning control process according to the first variation, the controller  30  detects three or more types of light incident from different directions using the detector  40 , while changing a direction of an optical axis of the detector  40 . The directions from which the detector  40  detects incident light may be chosen such that the detector  40  can detect light coming from multiple points on the inner wall of the processing vessel  11  each of which are evenly distributed in a circumferential direction of the processing vessel  11 . In the example of  FIG. 10 , five types of light each coming from a different direction is detected. Lines L 1  to L 5  illustrated in  FIG. 10  indicate optical axes of the detector  40 , and the detector  40  according to the first variation of the present embodiment detects five types of light incident along these five optical axes (L 1  to L 5 ). The first optical axis L 1  passes through the plasma generating region P. Intensity of light incident on the detector  40  along the first optical axis L 1  is used to measure (estimate) a state of a plasma, that is, the intensity of light incident on the detector  40  along the first optical axis L 1  corresponds to the above-described first plasma emission intensity. Meanwhile the second optical axes L 2  to L 5  extend from the detector  40  to the side wall of the processing vessel  11  without passing through the plasma generating region P. Intensity of light incident on the detector  40  along each of the second optical axes L 2  to L 5  is used to measure (estimate) states of the inner wall of the processing vessel  11 , that is, the intensity of light incident on the detector  40  along each of the second optical axes L 2  to L 5  corresponds to the second plasma emission intensity. In the description of the first variation, the intensity of light incident on the detector  40  along the first optical axis L 1  is also referred to as the first plasma emission intensity, and the intensity of light incident on the detector  40  along each of the second optical axes L 2  to L 5  is referred to as the second plasma emission intensity. It should be noted that multiple detectors  40  may be used to detect light coming from different directions, instead of changing a direction of the optical axis of a single detector  40 . 
     When the process of  FIG. 9  is started, wafer processing is performed in steps S 30  to S 34 . Next, the controller  30  acquires the measurement value of the first plasma emission intensity measured using light incident on the detector  40  along the first optical axis L 1  and the respective measurement values of the second plasma emission intensities measured using light incident on the detector  40  along the second optical axes L 2  to L 5  (step S 50 ). Also, in step S 50 , for each of the measurement values of the second plasma emission intensities, the controller  30  calculates a difference from the first plasma emission intensity. By performing step S 50 , multiple differences, each of which is a difference between the first plasma emission intensity and a corresponding one of the second plasma emission intensities, are calculated. 
     Next, the controller  30  determines whether or not at least one of the calculated differences exceeds the first threshold value (step S 52 ). If it is determined that none of the calculated differences exceeds the first threshold value, the controller  30  determines that it is not necessary to start dry cleaning. In such a case, the process returns to step S 30 , and the controller  30  repeats steps S 32  to S 34 , S 50 , and S 52 . 
     If it is determined that at least one of the calculated differences exceeds the first threshold value in step S 52 , the controller  30  executes dry cleaning (step S 40 ). Subsequently, in step S 54 , the controller  30  performs the same operation as step S 50 . That is, the controller  30  acquires the measurement value of the first plasma emission intensity and the respective measurement values of the second plasma emission intensities, and calculates, for each of the measurement values of the second plasma emission intensities, the difference from the first plasma emission intensity. 
     Next, in step S 56 , the controller  30  determines whether all of the differences calculated in step S 54  are within the normal range. If all of the calculated differences are within the normal range, it can be determined that the status of the side wall the processing vessel  11  is normal at all of the multiple points (points R 1  to R 5  in  FIG. 10 ) in the circumferential direction of the side wall of the processing vessel  11 . Thus, by performing step S 56 , the controller  30  can confirm that the wall condition is uniform in the circumferential direction. 
     The controller  30  continues the dry cleaning of step S 40  until all of the calculated differences become within the normal range. If it is determined that all of the calculated differences are within the normal range in step S 56 , the controller  30  determines whether or not a first predetermined period of time has elapsed since all the differences became within the normal range for the first time (step S 46 ). 
     The controller  30  repeats steps S 40 , S 54 , S 56 , and S 46  until it is determined that the first predetermined period of time has elapsed since all of the respective differences between the second plasma emission intensities and the first plasma emission intensity became within the normal range for the first time. 
     If it is determined that the first predetermined period of time has elapsed since all of the differences became within the normal range for the first time (YES in step S 46 ), the controller  30  terminates dry cleaning (step S 48 ). Then, the process returns to step S 30 , and the controller  30  repeats step S 30  and thereafter. 
     According to the cleaning process according to the first variation described above, it can be checked that the wall condition in the processing vessel  11  is uniform in the circumferential direction. Because uniformity of the state of the wall can be checked in the circumferential direction, dry cleaning can be started and terminated at more appropriate timings. Note that the points at which the state of the wall is measured (e.g., the points of the wall at which the optical axis of the detector  40  is directed) may not be uniform in the circumferential direction of the processing vessel  11 . For example, the points at which the optical axis is directed may be interspersed with respect to the side wall and a ceiling wall. Accordingly, the overall condition of the inner wall of the processing vessel  11  can be accurately detected. 
     Although the cleaning process has been described in the first variation, the above-described process of the first variation is not limited thereto. The above-described process of the first variation can be used for determining timing of termination of seasoning. For example, in step S 18  of  FIG. 6 , the seasoning may be terminated if it is determined that the second predetermined period of time has elapsed after all of the differences described in the first variation became within the normal range for the first time. 
     [Second Variation] 
     A seasoning process according to a second variation of the present embodiment will be described specifically, with reference to  FIG. 11 .  FIG. 11  is a flowchart illustrating the seasoning process according to the second variation of the present embodiment. Among steps of the seasoning process according to the second variation of  FIG. 11 , the same step numbers are assigned to the steps of performing the same process as that of  FIG. 6 . 
     In the seasoning process according to the second variation, when it is determined that seasoning starts (step S 10 ), the controller  30  performs seasoning in the processing vessel  11  (step S 12 ). 
     During the seasoning, the controller  30  acquires a measurement value of the first plasma emission intensity measured using light incident along the first optical axis L 1  from the detector  40  at a predetermined interval, and acquires respective measurement values of the second plasma emission intensities each measured using light incident along the second optical axes L 2  to L 5  at a predetermined interval. In step S 60 , the controller  30  calculates, for each of the acquired second plasma emission intensities, a difference from the acquired first plasma emission intensity. By performing step S 60 , multiple differences, each of which is a difference between the first plasma emission intensity and a corresponding one of the second plasma emission intensities, are calculated. 
     Next, in step S 62 , the controller  30  determines whether or not all of the calculated differences are within the normal range. If it is determined that at least one of the calculated differences is out of the normal range (NO in step S 62 ), the process returns to step S 12 , and the controller  30  continues the seasoning. Meanwhile, if it is determined that all of the calculated differences are within the normal range (YES in step S 62 ), the process proceeds to step S 18 . 
     In step S 18 , the controller  30  determines whether or not the second predetermined period of time has elapsed since all of the calculated differences became within the normal range for the first time. If it is determined that the second predetermined period of time is not elapsed since all of the calculated differences became within the normal range for the first time, the process returns to step S 12 , and the controller  30  continues the seasoning. 
     Meanwhile, if it is determined that the second predetermined period of time has elapsed after all of the calculated differences became within the normal range, the controller  30  terminates the seasoning of the processing vessel  11  (step S 20 ), to terminate the present seasoning process. 
     According to the seasoning process of the second variation described above, it can be checked that the wall condition of the wall in the processing vessel  11  is uniform in the circumferential direction. Because uniformity of the state of the wall can be checked in the circumferential direction, seasoning can be completed at more appropriate timings. 
     In the above-described embodiment and its variations, the first plasma emission intensity and the second plasma emission intensity are acquired by measuring intensity of a predetermined wavelength component in an optical spectrum of plasma light using OES. In the above-described embodiment and its variations, wall condition is determined based on a difference (subtraction) between the measured first plasma emission intensity and the measured second plasma emission intensity, but is not limited thereto. For example, a ratio (division) of the first plasma emission intensity to the second plasma emission intensity may be used to determine the wall condition. The ratio (division) of measured first and second plasma emission intensities can normalize a relationship between a state of the wall and a state of a plasma. This allows the state of the wall to be determined based on the normalized relationship between the state of the wall and the state of the plasma. 
     In the above described embodiment and its variations, a first predetermined period of time and a second predetermined period of time are used, but the number of dummy wafers may be used instead of time. For example, in a case in which a dummy wafer is loaded when performing seasoning and dry cleaning, in step S 18  of  FIG. 6  or  FIG. 11 , or in step S 46  of  FIG. 8  or  FIG. 9 , determination may be made based on the number of loaded dummy wafers. 
     The first and second plasma emission intensities may be measured simultaneously, or may be measured at generally consecutive timings. 
     Further, in the present embodiment and its variations, as examples of a conditioning process in the processing vessel  11 , seasoning and dry cleaning have been described, but the present invention is not limited thereto. The conditioning process may include pre-coating to coat a predetermined protective film (SiO 2  film) on the interior of the processing vessel  11 . In this case, the protective film may be formed by performing plasma processing under a different condition from the process condition when processing a wafer. 
     The plasma processing method and the plasma processing apparatus according to the embodiment disclosed herein are to be considered exemplary in all respects and not limiting. The above embodiment and its variations may be modified and enhanced in various forms without departing from the appended claims and the gist thereof. Matters described in the above embodiment and variations thereof may take other configurations to an extent not inconsistent, and may be combined to an extent not inconsistent. 
     The present disclosure is applicable to any types of plasma processing apparatus, such as an atomic layer deposition (ALD) type, a capacitively coupled plasma (CCP) type, an inductively coupled plasma (ICP) type, a radial line slot antenna type, an electron cyclotron resonance plasma (ECR) type, and a helicon wave plasma (HWP) type.