Patent Publication Number: US-10332760-B2

Title: Method for controlling plasma processing apparatus

Description:
CLAIM OF PRIORITY 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2013-024207, filed Feb.12, 2013, and Application No. 2013-112562, filed May 29, 2013, the entire contents of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for controlling a parallel plate plasma processing apparatus that manufactures semiconductor devices and Micro Electro Mechanical System (MEMS) devices. 
     2. Description of the Related Art 
     For micro-fabrication of grooves in a space width of 10 nm and aspect of 15 or more on stacked films such as silicon, silicon oxidize, and silicon nitride, micro-fabrication is performed mainly using a parallel plate plasma processing apparatus that generates plasma in a region sandwiched between an upper electrode and a lower electrode. For the parallel plate plasma source, a capacitive coupling plasma (CCP) apparatus is used as well as a magnetic field VHF plasma apparatus is used including a supply for a VHF wave of 200 MHz and a magnetic field generating coil. 
     This magnetic field VHF plasma apparatus has a structure below. The upper electrode of the magnetic field VHF plasma apparatus includes a function of emitting VHF waves for plasma generation. For an upper electrode member, a dielectric ceramic material such as silica, yttria, and sapphire glass or a material that an aluminum material or stainless steel material is coated with a dielectric ceramic material is used for a surface contacting plasma, from viewpoints of contamination and foreign substances. Moreover, the plasma generation distribution and the in-plane distribution of the etching rate can be controlled using a magnetic field from a magnetic field generating coil. A radio frequency bias can be applied to the lower electrode on which a wafer is placed for anisotropy etching. See Japanese Patent Application Laid-Open Publication No. 2007-59567. 
     On the other hand, in the CCP etching apparatus, a plasma processing apparatus is disclosed for improving uniformity, which includes a circuit that adjusts electrical characteristics (impedance) on the counter electrode side on which a bias frequency is applied so as to prevent an electric current flowing into the counter electrode side from becoming the maximum. See Japanese Patent Application Laid-Open Publication No. 2011-82180, which discloses a control method in which a bias current is adjusted to a half of the maximum electric current or more. 
     SUMMARY OF THE INVENTION 
     In order to perform highly uniform etching with much less contamination using a magnetic field VHF plasma etching apparatus including a dielectric ceramic on the upper electrode, the inventors conducted control described in Japanese Patent Application Laid-Open Publication No. 2011-82180 in which a counter bias control mechanism is mounted on the upper electrode side, including a resonance coil that cancels reactance caused by the electrostatic capacitance of the dielectric ceramic and a variable capacitance. As a result, it was revealed that the following problems arise in that in the case where a multi-layer film is etched in multiple steps, the etching conditions are varied in the individual steps, and thus the magnitude of the variable capacitance to resonate and the absolute value of a counter bias current are changed, and in that when a preliminary study is conducted before processing as the measures for these changes, CoO is increased due to the use of a dummy wafer and a preparation is prolonged until processing time, for example. 
     Moreover, it was revealed that in the case of using control described in Japanese Patent Application Laid-Open Publication No. 2011-82180 in an end point determination step, since the counter bias current and the resonating reactance themselves are changed in the step, the bias current value goes out of the resonance point in the midway point of the change, the plasma distribution is changed, and the in-plane distribution of the substrate error selection ratio is degraded. 
     A first object of the present invention is to provide a method for controlling a plasma processing apparatus that eliminates a preliminary study on a resonance point while maintaining a low contamination and a high uniformity even in multi-step etching. Moreover, a second object of the present invention is to provide a method for controlling a plasma processing apparatus that follows changes in the resonance point or changes near the set resonance point and enables highly uniform etching of a multi-layer film even in a so-called transient state in which a counter bias current or plasma impedance is changed in an ignition step, an end point determination step, and other steps. 
     In order to solve the problems, configurations and process procedures described in the appended claims, for example, are adopted. 
     The present specification includes a plurality of means for solving the problems. One example is a method for controlling a plasma processing apparatus including a plasma processing chamber configured to plasma-process an object to be processed, a first flat electrode configured to emit a radio frequency into the plasma processing chamber, a first radio frequency power supply configured to supply radio frequency power to the first electrode, a second electrode opposite to the first electrode and on which the object to be processed is placed, a second radio frequency power supply configured to supply radio frequency power to the second electrode, and a control mechanism configured to control a radio frequency current carried through the first electrode or a radio frequency voltage applied to the first electrode, the method including: a first step of setting a reactance of a variable element included in the control mechanism to an initial value; a second step of detecting the radio frequency current or the radio frequency voltage; and a third step of setting the reactance of the variable element to a reactance value so that the radio frequency current takes a maximum value or the radio frequency voltage takes a maximum value and fixing the reactance of the variable element to the set reactance value. 
     Moreover, another example is a method for controlling a plasma processing apparatus including a plasma processing chamber configured to plasma-process an object to be processed, a first flat electrode configured to emit a radio frequency into the plasma processing chamber, a first radio frequency power supply configured to supply radio frequency power to the first electrode, a second electrode opposite to the first electrode and on which the object to be processed is placed, a second radio frequency power supply configured to supply radio frequency power to the second electrode, and a control mechanism configured to control a radio frequency current carried through the first electrode or a radio frequency voltage applied to the first electrode, the method including: a first step of detecting a phase difference between a radio frequency current carried through the second electrode and a radio frequency current carried through the first electrode or a phase difference between a radio frequency voltage applied to the second electrode and a radio frequency voltage applied to the first electrode; and a second step of controlling a reactance of a variable element included in the control mechanism so that the detected phase difference takes a phase difference value matched with a maximum value of the radio frequency current carried through the first electrode or a maximum value of the radio frequency voltage applied to the first electrode. 
     According to the present invention, it is possible to provide a method for controlling a plasma processing apparatus that eliminates a preliminary study on a resonance point while maintaining a low contamination and a high uniformity even in multi-step etching. 
     Moreover, it is possible to provide a method for controlling a plasma processing apparatus that follows changes in the resonance point or changes near the set resonance point and enables highly uniform etching of a multi-layer film even in a so-called transient state in which a counter bias current or plasma impedance is changed in an ignition step, an end point determination step, and other steps. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become fully understood from the detailed description given hereinafter and the accompanying drawings, wherein: 
         FIG. 1  is a schematic cross sectional view of the overall structure of a dry etching apparatus (a magnetic field VHF dry etching apparatus) for use in performing a method for controlling a plasma processing apparatus according to a first embodiment of the present invention; 
         FIG. 2  is a circuit block diagram of a counter bias control mechanism of the dry etching apparatus illustrated in  FIG. 1 ; 
         FIG. 3  is a flowchart for explaining control of the method for controlling a plasma processing apparatus according to the first embodiment of the present invention; 
         FIG. 4  is a graph of the dependence of a counter bias current on a variable capacitance in the dry etching apparatus illustrated in  FIG. 1 ; 
         FIG. 5  is a graph of the dependence of the phase difference between a counter bias current and a radio frequency bias current on a variable capacitance in the dry etching apparatus illustrated in  FIG. 1 ; 
         FIG. 6  is a flowchart for explaining control of a method for controlling a plasma processing apparatus according to a second embodiment of the present invention; 
         FIG. 7  is a schematic cross sectional view of the overall structure of another dry etching apparatus (a CCP etching apparatus) for performing the method for controlling a plasma processing apparatus according to the first and second embodiments of the present invention; 
         FIG. 8  is a schematic cross sectional view of the overall structure of a dry etching apparatus (a magnetic field VHF dry etching apparatus) for use in performing a plasma processing method according to a third embodiment of the present invention; 
         FIG. 9  is a circuit block diagram of a counter bias control mechanism of the dry etching apparatus illustrated in  FIG. 8 ; 
         FIG. 10  is a graph of the dependence of a counter bias current on a variable capacitance in the dry etching apparatus illustrated in  FIG. 8 ; 
         FIG. 11  is a graph of the in-plane distribution of the oxidization film etching rate on a shower plate when a counter bias control mechanism is resonating and not resonating in the dry etching apparatus illustrated in  FIG. 8 ; 
         FIG. 12  is a time sequence of changes in monitor values in cleaning and the control of a variable capacitor in the dry etching apparatus illustrated in  FIG. 8 ; 
         FIG. 13  is a block diagram of an end point determination circuit for plasma processing according to the third embodiment of the present invention; 
         FIG. 14  is a graph of the dependence of the oxidization film etching rate on the RF bias power at the center point of the shower plate when the counter bias control mechanism is resonating and not resonating in the dry etching apparatus illustrated in  FIG. 8 ; and 
         FIG. 15  is a cross sectional view of a dry etching apparatus with no electromagnet for performing another plasma processing method according to the third embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     A first embodiment of the present invention will be described with reference to  FIGS. 1 to 4 . First, a plasma processing apparatus mounted with a counter bias control mechanism that embodies a bias current control method in a method for controlling a plasma processing apparatus according to the embodiment will be described.  FIG. 1  is a vertical cross sectional view of a parallel plate magnetic field VHF dry etching apparatus. 
     A vacuum container of the dry etching apparatus includes an etching chamber  108  for a plasma processing chamber, an earth internal cylinder  107 , a silica top plate  111 , a VHF radiation antenna  115 , a vacuum pump, and a pressure control valve (both of them are not illustrated in  FIG. 1 ). 
     Etching gases pass through a mass flow controller and a stop valve (both of them are not illustrated in  FIG. 1 ), and pass through a gas inlet port A  109  and a gas inlet port B  112 . The gases are distributed while preventing the gases from being mixed with each other using a gas distribution plate  114 , and introduced from regions of a shower plate  116  concentrically divided into two parts into the etching chamber. The gases thus introduced are dissociated from each other by energy of electromagnetic waves applied from a plasma generating unit, and plasma is generated and maintained. 
     This plasma generating unit includes a source power supply  101  of a VHF wave of 200 MHz, a source electromagnetic wave matching unit  102 , and a magnetic field generating unit formed of an electromagnet A  105  and an electromagnet B  106 . These two electromagnets are used to uniformize the plasma generation distribution. The generated magnetic field is at 10 mT or less near the shower plate  116 . VHF waves oscillated from the source power supply  101  pass through the source electromagnetic wave matching unit  102 , and are introduced into the VHF radiation antenna  115  at a position opposite to a wafer stage  120 . The VHF radiation antenna  115  is electrically isolated from the etching chamber  108  using the silica top plate  111 . 
     An Si wafer (an object to be processed)  117  is placed on the wafer stage  120 , including a stack of etched materials and mask materials such as a silicon oxide film, silicon nitride film, Poly-Si (polysilicon) film, resist film, anti-reflective film, TiN film, tungsten film, Ta compound film, and Hf oxide film. The wafer stage  120  includes a focus ring  118  and a susceptor  119  in a ring shape disposed as covering the outer circumferential side and the side wall of the surface on which the Si wafer  117  is placed. The wafer stage  120  can control a plurality of portions on the wafer stage  120  at different predetermined temperatures using a plurality of temperature control units, for example, (not illustrated in  FIG. 1 ). The wafer stage  120  applies a direct current voltage ranging from −2,000 to +2,000 V generated using an electrostatic chuck (ESC) direct current power supply  122  to electrostatically chuck the Si wafer  117  in etching processing, fills He of an excellent heat transfer efficiency in a gap between the Si wafer  117  and the wafer stage  120 , and controls the back surface pressure of the Si wafer  117 . For the shower plate  116 , silica or yttria was used, which have corrosion resistance against gasses and do not cause foreign substance emission. Since the shower plate  116  is in intimate contact with the gas distribution plate  114  and the VHF radiation antenna  115  using screws, for example, an excessive temperature increase can be suppressed by adjusting the temperature of a cooling medium for the VHF radiation antenna  115 . 
     The wafer stage  120  is connected to an RF bias matching unit  121  and to a 4-MHz RF bias power supply  123  that leads ions from plasma to the Si wafer  117  and controls ion energy. 
     Such an RF bias power supply including a time modulation (sometimes denoted as TM) function was used for the RF bias power supply  123 , in which power can be outputted in a range of about one watt at the lowest to about two kilowatts at the maximum equivalent to the emission of continuous sine waves to an object to be processed in a diameter of 12 inches and on-off modulation is performed in a range of one hertz to ten kilohertz in order to obtain the effects of a reduction in charge up damage (electron shading) and improved vertical processability. 
     A radio frequency bias current applied to the wafer stage  120  propagates through the inside of plasma toward the earth internal cylinder  107  disposed as an earth on the inner wall of the etching chamber  108  through a plasma sheath on the Si wafer  117 . For the earth internal cylinder  107 , such a conductive material is used as a conductive material of a low contamination or as a conductive material including a thermal sprayed material of a low reactivity with etching plasma through which a radio frequency passes, in order to reduce contamination in the apparatus and foreign substances. 
     In the parallel plate magnetic field VHF etching apparatus, in the embodiment, a counter bias control mechanism  104  is mounted through a filter unit  103  in order that a bias is transmitted to the VHF radiation antenna  115  side opposite to the wafer stage  120  to control the degree of confinement of the bias electric field for improving etching uniformity. The filter unit  103  includes a highpass filter (HPF) that prevents an RF bias of 4 MHz and the third-order harmonic of the RF bias from passing on the source power supply side and a lowpass filter (LPF) that flows only an RF bias frequency to the earth side. It is noted that a reference numeral  110  denotes a cooling medium inlet, a reference numeral  113  denotes a cooling medium outlet, a reference numeral  124  denotes a radio frequency bias current detecting unit, a reference numeral  125  denotes a wafer stage elevating mechanism, a reference numeral  126  denotes a silica ring, a reference numeral  127  denotes a resonance control circuit, a reference numeral  128  denotes a yoke, a reference numeral  131  denotes an EPD (End Point Detector) window, and a reference numeral  133  denotes a shield plate. 
       FIG. 2  is a diagram of the configuration of the counter bias control mechanism  104 . The counter bias control mechanism  104  is configured of a serial resonant portion formed of a resonant coil  201  of low resistance that hardly generates heat even at the maximum electric current at an RF bias of 4 MHz and a variable capacitor  202  having a moderate withstand voltage, a counter bias current detecting circuit  203 , and a resonance control circuit  127 . In consideration of the electrostatic capacitance (C sp ) of the silica shower plate  116  and the electrostatic capacitance (C sh ) of a sheath formed on the shower plate, the inductance (L) of the resonant coil  201  and the electrostatic capacitance (C v ) of the variable capacitor  202  are selected using the relationship between Equations (1) to (3). 
     
       
         
           
             
               
                 
                   
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     Here, ω is the angular velocity of the RF bias frequency. X v  is in the relationship in Equation (2) where the capacitance is C v  in the case where the variable reactance element is a capacitor, whereas X v  is in the relationship in Equation (3) where the inductance is L v  in the case where the variable reactance element is a coil. 
     
       
         
           
             
               
                 
                   
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     Moreover, a plurality of sets of a harmonic short circuit coil  204  and a harmonic short circuit fine tune capacitor  205  according to the harmonic order is inserted in parallel with a circuit formed of the resonant coil  201  and the variable capacitor  202 , and the impedance of a harmonic component generated when passing through the plasma sheath on the VHF radiation antenna  115  can also be reduced, so that etching can be uniformized for wider plasma conditions. Furthermore, the electric current values of a plurality of harmonic components are monitored using a harmonic current detection circuit  207 , so that information about the plasma density and the electron temperature can be obtained as well, and a change in the state of the apparatus can be detected more accurately. It is noted that a reference numeral  206  denotes an automatic matching unit, and a reference numeral  209  denotes an automatic harmonic matching unit. 
     The embodiment relates to a bias current control method using the counter bias control mechanism  104  disposed on the parallel plate plasma processing apparatus thus configured.  FIG. 3  is a control flow for explaining a bias current control method of a method for controlling a plasma processing apparatus according to the embodiment. Moreover,  FIG. 4  is the dependence of the counter bias current on the variable capacitor capacitance. When the etching sequence is started (S 1 ), an apparatus control PC sends signals of a preset position  403  and a target delta value  406  for the variable capacitor (the variable element)  202  to the resonance control circuit  127 , and the variable capacitor  202  is adjusted to the preset position (S 2 ). At this time, in the case where the apparatus control PC does not instruct the automatic control mode, the variable capacitor  202  is fixed at the preset position  403  in etching. On the other hand, in the case where the apparatus control PC instructs the automatic control mode, the RF bias power supply  123  outputs power (S 3 ). Automatic control is then started from a point at which the antenna bias current exceeds a threshold at the counter bias current detecting circuit  203  (S 4 ), and the variable element  202  starts the operation toward a resonance point  405  (S 5 ). 
       FIG. 4  is measured data of a typical tendency of the counter bias current showing manners of the counter bias current and the variable capacitance. It is shown that since the point at which the bias current becomes the maximum is the resonance point, the counter bias current value takes the maximum value with respect to the capacitance of the variable capacitor  202 . Moreover, the capacitance of the variable capacitor at the maximum value and the maximum value of the counter bias current are changed in the range of about 50 pF when the electrical capacitance (C sh ) of the sheath formed on the shower plate is changed, that is, when the plasma conditions (the output power of the source power supply  101 , the processing pressure, and the power of the RF bias power supply  123 , for example) are changed. Furthermore, when the electrostatic capacitance becomes greater than that at the resonance point, the bias current is suddenly reduced, and the etching rate distribution is similarly suddenly degraded as well. Thus, preferably, automatic control is started as the electrostatic capacitance at the preset position  403  is set lower than that at the resonance point  405 . It is noted that a reference numeral  401  denotes the peak-to-peak current of the counter bias, and a reference numeral  404  denotes a counter bias current value when automatic control is finished. 
     Therefore, in the embodiment, the preset position  403  is selected at which the electrostatic capacitance is smaller than the electrostatic capacitance at the resonance point  405  (greater as reactance) and the bias current is a threshold current or more in starting. When the bias current exceeds the set threshold (S 4 ), the resonance control circuit  127  changes the capacitance of the variable capacitor in the direction in which the bias current is increased. The electrostatic capacitance position at which the bias current is turned into a reduction is stored as the resonance capacitance (S 6 ), the capacitance is moved from the position to the capacitance by the set target delta value  406  (S 7 ), and the capacitance is fixed at the position in etching processing (S 8 ). After that, the radio frequency bias is turned off, the capacitance of the variable capacitor  202  is reset when the counter bias current is lower than the set threshold (S 10 ), and a series of the operations is finished (S 11 ). 
     The description above is the operation in the first step in which plasma discharge is intermitted in every step. In the case where the etching conditions are changed as plasma discharge is continued, the variable capacitor  202  is fixed, and automatic matching is finished (S 8 ), and a trigger signal outputted at the timing at which the process goes to the subsequent step is then received from the apparatus control PC (S 9 ). In the case where discharge is continued, the variable element is adjusted to the preset value in the in the subsequent step (S 2 ), and the automatic control flow is again started in the midway point. In the restarting, the preset value in the step in which discharge is continued is set to a value smaller than the capacitance of the variable capacitor at the resonance point, so that it is possible to improve the degradation of uniformity and stability after discharge is continued. 
     Moreover, in the application of the TM bias, the repetition frequency for turning on and off is synchronized with the timing of detecting the counter bias current in the counter bias current detecting circuit  203  for control only using values when turned on, so that automatic control is made possible. 
     According to the embodiment, in the parallel plate plasma apparatus using a dielectric material such as silica for the shower plate, even in the case where the etching process configured of multiple steps under plasma conditions different from each other and the application of the conditions are performed for a first time, it is possible to eliminate the necessity of studying the maximum values of the resonance point and the bias current in advance, to reduce malfunctions caused by changing the processing condition, to shorten turn around time (TAT), and to improve the reproducibility of uniformity. At this time, when the initial value of the variable capacitor  202  is set to a value of the capacitance smaller than the resonating capacitance, automatic control can be performed without degrading uniformity in searching for the resonance point. 
     As described above, the embodiment is described in which the counter bias current detecting circuit  203  detects the counter bias current for automatic control. However, also in the case of monitoring the voltage across the earth and the point on the passage through which the counter bias current passes (a reference numeral  208  in  FIG. 2 , for example) or monitoring the voltage across the resonant coil  201 , the behaviors of a peak-to-peak voltage  402  of the counter bias and the variable capacitor  202  are identical to the behavior of the counter bias current as illustrated in  FIG. 4 . Therefore, similar control can be performed even though the counter bias voltage value is used for the monitor signal. 
     Moreover, it may be fine that an impedance monitor is inserted between the point  208  and the resonant coil  201  in  FIG. 2  and the variable capacitor  202  is controlled based on the detected impedance information. In this case, it may be fine that the point at which the reactance that is an imaginary component of the impedance is zero is searched instead of searching for the maximum value of the bias current. In the application of the TM bias, control can be performed in which the impedance monitor is synchronized with the timing at which the bias current is turned on for control using the impedance when the bias current is turned on. 
     The case is described in the control flowchart of  FIG. 3  where the trigger to start automatic matching is the timing at which the bias current exceeds the threshold. However, when the trigger is changed to a trigger signal outputted from the etching apparatus side, automatic control can be started after finishing the high transient phenomenon of the startup of the power supply, for example, so that malfunctions caused by the transient phenomenon in ignition can be suppressed. Similarly, when it is permitted to separately set waiting time until starting the operation after the trigger signal is inputted from the apparatus side or after the bias current exceeds the threshold also in the counter bias control mechanism, the apparatus can meet all the process conditions. 
     Also for the method for controlling the harmonic short circuit fine tune capacitor  205 , similar control is performed as the bias of the principal component using the monitor result at the harmonic current detection circuit  207  and the automatic harmonic matching unit  209 , so that the uniformity can be further improved. 
     Second Embodiment 
     A second embodiment of the present invention will be described with reference to  FIGS. 5 to 7 . It is noted that points described in the first embodiment but not described in the second embodiment can also be applied to the second embodiment unless otherwise specified. 
     In the embodiment, an embodiment will be described below in which even in the case where an electrostatic capacitance to resonate with a bias current changes in the steps such as the end point determination step, the changes can be followed. In order to implement the embodiment, in addition to monitoring a counter bias current, a radio frequency bias current detecting unit  124  detects phase information about a radio frequency bias current, and inputs the information to a resonance control circuit  127 . 
     In this operation, a counter bias current detecting circuit  203  also acquires phase information about the counter bias current, and inputs the information to the resonance control circuit  127 . The resonance control circuit  127  calculates the difference between the phase of the counter bias current and the phase of the radio frequency bias current oscillated in plasma, and controls the variable capacitance based on the result. 
       FIG. 5  is a graph of the dependence of the phase difference between the counter bias current and the radio frequency bias current on the variable capacitance. It was newly discovered that the position at which the bias current becomes the maximum is the position at which the phase difference is at an angle of −90°. Since it is known by experiment that this phase difference is constant regardless of plasma conditions, the monitor value of this phase difference is controlled so as to be matched with the target value, and transient changes can be followed as well. It is noted that a reference numeral  501  denotes the peak-to-peak current of the counter bias, and a reference numeral  503  denotes a phase difference in resonance. 
       FIG. 6  is a flowchart of phase difference detection according to the embodiment based on this principle. The process is as described in the first embodiment until starting automatic control (Steps S 12  to S 14  correspond to Steps S 1  to S 3 ). 
     When a bias current is detached to start control (S 15 ), a variable capacitor  202  is adjusted for the phase difference set on the etching apparatus side based on the relationship in  FIG. 5  (S 16 ). At this time, it is necessary to place a preset position  403  within −80 pF from the resonance point. This is because the phase difference is always reduced as the capacitance of the variable capacitor is increased in the region in which the preset position is placed within 80 pF. 
     According to the relationship illustrated in  FIG. 5 , in the case where the monitored phase difference is smaller than the permitted value of the set value, the variable capacitor  202  is reduced, whereas in the case where the monitored phase difference is greater than a set value  502 , the variable capacitor  202  is increased. The capacitance of the variable capacitor  202  is then changed in such a way that the monitor value falls within the permitted value of the set value (S 17 ). 
     In this control, as similar to the case of the first embodiment, since the capacitance smaller than the capacitance at a resonance point  405  has a small amount of change in uniformity for the phase difference at the set value  502 , the phase difference is set greater than that at the point at an angle of −90°, which can provide more stable performance against a variation over time, for example. Subsequently, in Step  18  (S 18 ), it is confirmed whether to continue discharge in the subsequent step. In the case where it is necessary to continue discharge in the subsequent step, the process is returned to Step  13  (S 13 ). In the case where it is unnecessary to continue discharge in the subsequent step, it is confirmed whether the bias current is smaller than the set threshold in Step  19  (S 19 ). In the case where the bias current is greater than the set threshold, the process is returned to Step  14  (S 14 ). In the case where the bias current is smaller than the set threshold, the process is finished (S 20 ). 
     As described above, according to the embodiment using phase difference detection, it is possible to directly reach the target value without exceeding the absolute value of a counter bias current or the variable capacitance of resonance, which are varied depending on the plasma conditions, and to automatically follow changes in the bias resonance point and the resonance position (the plasma impedance) as in determining the end point. It is noted that the method is also applicable to multistep etching. 
     It is noted that as similar to the first embodiment, the control method is similar when control is performed not only by the phase difference between the bias currents but also by the phase difference between the bias voltages. However, the phase difference at the resonance point is changed depending on the position of the voltage to be measured, and the phase difference is not always at an angle of −90°, so that it is necessary to check phase differences in advance according to the configuration of an apparatus for use. 
     As described above, for the method descried in the first and second embodiments, the example is described in which the source power supply  101  at 200 MHz and the RF bias power supply of magnetic field VHF plasma at 4 MHz are mounted. However, the method is also applicable to a parallel plate apparatus (a so-called CCP apparatus) with no magnetic field as illustrated in  FIG. 7 . The apparatus in  FIG. 7  is configured in which a source power supply  101  is connected on the wafer stage side and a shower plate  116  to be the surface of a counter earth electrode  701  is formed of a dielectric material. The method is similarly applicable by connecting a counter bias control mechanism  104  for a source power supply on the counter earth electrode  701  side. 
     Third Embodiment 
     A third embodiment of the present invention will be described with reference to  FIGS. 8 to 15 . It is noted that points described in the first or second embodiment but not described in the third embodiment can also be applied to the third embodiment unless otherwise specified. First, a plasma processing apparatus mounted with a counter bias control mechanism that embodies a cleaning method of a plasma processing method according to the embodiment will be described.  FIG. 8  is a vertical cross sectional view of a parallel plate magnetic field VHF dry etching apparatus. 
     A vacuum container of the dry etching apparatus includes an etching chamber  108  for a plasma processing chamber, an earth internal cylinder  107 , a silica top plate  111 , a VHF radiation antenna  115 , a vacuum pump, and a pressure control valve (both of them are not illustrated in  FIG. 8 ). 
     Etching gases pass through a mass flow controller and a stop valve (both of them are not illustrated in  FIG. 8 ), and pass through a gas inlet port A  109  and a gas inlet port B  112 . The gases are distributed while preventing the gases from being mixed using a gas distribution plate  114 , and introduced from regions of a shower plate  116  concentrically divided into two parts into the etching chamber. The gases thus introduced are dissociated from each other by energy of electromagnetic waves applied from a plasma generating unit, and plasma is generated and maintained. 
     This plasma generating unit includes a source power supply  101  of a VHF wave of 200 MHz, a source electromagnetic wave matching unit  102 , and a magnetic field generating unit formed of an electromagnet A  105  and an electromagnet B  106 . These two electromagnets are used to uniformize the plasma generation distribution. The generated magnetic field is at 10 mT or less near the shower plate  116 . VHF waves oscillated from the source power supply  101  pass through the source electromagnetic wave matching unit  102 , and are introduced into the VHF radiation antenna  115  at a position opposite to a wafer stage  120 . The VHF radiation antenna  115  is electrically isolated from the etching chamber  108  using the silica top plate  111 . 
     An Si wafer  117  is placed on the wafer stage  120 , including a stack of etched materials and mask materials such as a silicon oxide film, silicon nitride film, Poly-Si (polysilicon) film, resist film, anti-reflective film, TiN film, tungsten film, Ta compound film, and Hf oxide film. The wafer stage  120  includes a focus ring  118  and a susceptor  119  in a ring shape disposed as covering the outer circumferential side and the side wall of the surface on which the Si wafer  117  is placed. The wafer stage  120  can control a plurality of portions on the wafer stage 120 at different predetermined temperatures using a plurality of temperature control units, for example, (not illustrated in  FIG. 8 ). The wafer stage  120  applies a direct current voltage ranging from −2,000 to +2,000 V generated using an electrostatic chuck (ESC) direct current power supply  122  to electrostatically chuck the Si wafer  117  in etching processing, fills He of an excellent heat transfer efficiency in a gap between the Si wafer  117  and the wafer stage  120 , and controls the back surface pressure of the Si wafer  117 . For the shower plate  116 , silica, sapphire, or yttria was used, which have corrosion resistance against gasses and do not cause foreign substance emission. Since the shower plate  116  is in intimate contact with the gas distribution plate  114  and the VHF radiation antenna  115  using screws, for example, an excessive temperature increase can be suppressed by adjusting the temperature of a cooling medium for the VHF radiation antenna  115 . 
     The wafer stage  120  is connected to an RF bias matching unit  121  and to a 4-MHz RF bias power supply  123  that leads ions from plasma to the Si wafer  117  and controls ion energy. 
     Such an RF bias power supply including a time modulation (sometimes denoted as TM) function was used for the RF bias power supply  123 , in which power can be outputted in a range of about one watt at the lowest to about four kilowatts at the maximum equivalent to the emission of continuous sine waves to an object to be processed in a diameter of 12 inches and on-off modulation is performed in a range of one hertz to ten kilohertz in order to obtain the effects of a reduction in charge up damage (electron shading) and improved vertical processability. 
     A radio frequency bias current applied to the wafer stage  120  propagates through the inside of plasma toward the earth internal cylinder  107  disposed as an earth on the inner wall of the etching chamber  108  through a plasma sheath on the Si wafer  117 . For the earth internal cylinder  107 , such a conductive material is used as a conductive material of a low contamination or as a conductive material including a thermal sprayed material of a low reactivity with etching plasma through which a radio frequency passes, in order to reduce contamination in the apparatus and foreign substances. 
     In the parallel plate magnetic field VHF etching apparatus, in the embodiment, a counter bias control mechanism  104  is mounted through a filter unit  103  in order that a bias is transmitted to the VHF radiation antenna  115  side opposite to the wafer stage  120  to control the degree of confinement of the bias electric field for improving etching uniformity. The filter unit  103  includes a highpass filter (HPF) that prevents an RF bias of 4 MHz and the third-order harmonic of the RF bias from passing on the source power supply side and a lowpass filter (LPF) that flows only an RF bias frequency to the earth side. It is noted that a reference numeral  110  denotes a cooling medium inlet, a reference numeral  113  denotes a cooling medium outlet, a reference numeral  124  denotes a radio frequency bias current detecting unit, a reference numeral  125  denotes a wafer stage elevating mechanism, a reference numeral  126  denotes a silica ring, a reference numeral  127  denotes a resonance control circuit, a reference numeral  128  denotes a yoke, a reference numeral  131  denotes an EPD (End Point Detector) window, and a reference numeral  133  denotes a shield plate. The radio frequency bias current detecting unit  124  may be disposed in the RF bias matching unit  121 . 
       FIG. 9  is a diagram of the configuration of the counter bias control mechanism  104 . The counter bias control mechanism  104  is configured of a serial resonant portion formed of a resonant coil  201  and a variable capacitor  202  having a moderate withstand voltage, a counter bias current detecting circuit  203 , and a resonance control circuit  127 . In consideration of the electrostatic capacitance (C sp ) of the silica shower plate  116  and the electrostatic capacitance (C sh ) of a sheath formed on the shower plate, the inductance (L) of the resonant coil  201  and the electrostatic capacitance (C v ) of the variable capacitor  202  are selected using the relationship between Equations (1) to (3) described in the first embodiment. 
     Moreover, a plurality of sets of a harmonic short circuit coil  204  and a harmonic short circuit fine tune capacitor  205  according to the harmonic order is inserted in parallel with a circuit formed of the resonant coil  201  and the variable capacitor  202 , and the impedance of a harmonic component generated when passing through the plasma sheath on the VHF radiation antenna  115  can also be reduced, so that etching can be uniformized for wider plasma conditions. Furthermore, the electric current values of a plurality of harmonic components are monitored using a harmonic current detection circuit  207 , so that information about the plasma density and the electron temperature can be obtained as well, and a change in the state of the apparatus can be detected more accurately. It is noted that a reference numeral  206  denotes an automatic matching unit, a reference numeral  208  denotes a voltage measurement point, and a reference numeral  209  denotes an automatic harmonic matching unit. 
     The embodiment relates to a plasma cleaning method using the counter bias control mechanism  104  disposed on the parallel plate plasma processing apparatus thus configured. Plasma cleaning is necessary for stabilizing mass production in the etching process in the process step of removing etching reaction products attached in the etching chamber in etching processing. Plasma cleaning is appropriately inserted between individual wafers or lots after the etching process. 
     For example, for a cleaning gas in etching Si using Cl 2  or HBr, such a gas is used that oxygen or nitrogen, for example, is mixed in a gas to supply fluorine such as SF 6 , NF 3 , and CF 4 . For a cleaning gas in etching SiO 2  or SiN using a fluorocarbon gas, such a gas is used that O 2  or N 2  is mixed, or H is mixed in some case. For a cleaning gas in etching Al, Ti, or Hf, for example, a gas such as Cl 2 , HCl, and HBr is used. 
       FIG. 10  is measured data that a change in a counter bias current Ipp  302  is measured at the counter bias current detecting circuit  203  when the electrostatic capacitance of the variable capacitor  202  in the counter bias control mechanism  104  is changed. It is shown that since the point at which the bias current becomes the maximum is the resonance point, the counter bias current value takes the maximum value with respect to the capacitance of the variable capacitor  202 . Moreover, the capacitance of the variable capacitor at the maximum value and the maximum value of the counter bias current are changed in the range of about 50 pF when the electrical capacitance (C sh ) of the sheath formed on the shower plate is changed, that is, when the plasma conditions (the output power of the source power supply  101 , the processing pressure, and the power of the RF bias power supply  123 , for example) are changed. A reference numeral  301  denotes a dissonance point, and a reference numeral  303  denotes a resonance point. Furthermore, in a graph in  FIG. 10 , although a counter bias voltage Vpp  304  is also plotted at the voltage measurement point  208  in the counter bias control mechanism, the behavior is matched with the behavior of the counter bias current Ipp  302 . Thus, in the following, the description will be given as the case where the counter bias voltage Vpp  304  is detected. 
       FIG. 11  is the in-plane distribution of the rate of an oxide film attached on the shower plate  116  when the counter bias control mechanism  104  is resonating (a reference numeral  410  in  FIG. 11 ) and when the counter bias control mechanism  104  is not resonating (a reference numeral  420  in  FIG. 11 ) where a SF 6 /O 2  gas is at 8 Pa and the RF bias power is at 100 W. It was found that the etching rate of an oxide film in simulating the wearing out of the silica shower plate is about 20 nm/min in the plane on average in dissonance whereas the etching rate is increased twice or more at the resonance point as the oxide film rate is about 45 nm/min. 
     This is because the counter bias control mechanism  104  is resonated to reduce the reactance on the VHF radiation antenna  115  side, so that the ion current and the electron current are accelerated from plasma in the sheath and the currents flow in. With the use of this principle, the counter bias control mechanism  104  can be resonated in cleaning to cause an ion assist reaction on the silica shower plate  116 , so that the cleaning rate can be dramatically improved. 
     The timing chart of the cleaning method for the dry etching apparatus illustrated in  FIG. 8  to which the present invention is applied will be described with reference to  FIG. 12 .  FIG. 12  is a sequence chart of the measured results of the time variation of light emission intensity  511  at a wavelength of 440 nm measured through the EPD window  131 , an opening degree  512  of the pressure control valve (not illustrated in  FIG. 8 ), an RF bias voltage Vpp  513  detected at the RF bias matching unit  121 , and a counter bias voltage Vpp  514  detected at the voltage measurement point  208  in the counter bias control mechanism under cleaning processing after etched using a fluorocarbon gas and the manner of control of the variable capacitor  202  in the counter bias control mechanism. The following is the cleaning conditions, where O 2  is at 800 ccm, a pressure is at 4 Pa, the output of the source power supply  101  is at 800 W, and the output of the RF bias power supply  123  is at 1,000 W. 
     When the plasma light emission intensity  511 , the pressure control valve the position  512 , or the control valve opening degree is fixed, which are previously existing means for detecting the cleaning end point, relatively long time constants are observed for a change in the pressure, the RF bias voltage Vpp  513 , or the time variation of the plasma impedance detected on the RF bias side, whereas the time constant for the counter bias voltage Vpp  514  used in the present invention is short. This is because the cleaning end points are detected on the entire boundary contacting plasma in the previously existing detection method, whereas the time variation of the counter bias voltage Vpp  514  is short because the cleaning end point is detached on the boundary surface on the shower plate  116  side in the entire boundary. 
     Therefore, an end point  515  for removal of the attachment on the shower plate is determined at time at which the absolute value of the amount of change Vpp (A)−Vpp (B) in the counter bias voltage Vpp value between time A and time B becomes smaller than the set value for a specified number of times or more or determined at the inflection point of the counter bias voltage Vpp value (at the point at which the secondary difference of Vpp reaches zero). After a lapse of preset over cleaning time  519  from the timing, the capacitance of the variable capacitor  202  of the counter bias control mechanism  104  is adjusted from a variable capacitor position  517  in resonance to a position  518  for the variable capacitor in dissonance. 
     As a result of the control, the monitor value of the counter bias voltage Vpp is changed as a dotted line (a dotted line  520  in  FIG. 12 ). With this manipulation, the conditions can be changed in such a way that the conditions in which the cleaning effect of the shower plate due to an ion assist reaction is maximized are first used and then the conditions in which the degree of the wearing out of the shower plate is reduced in an ion impact are used, so that it is possible to reduce the frequency of replacing the shower plate, which enables a reduction in CoC and the extension of MTBM. It is noted that a reference numeral  516  is the end point of chamber cleaning. 
     Although a larger amount of change from the resonance point to the dissonance point is preferable, this is varied depending on the dielectric constant and thickness of the shower plate  116  and the electrical passage from the VHF radiation antenna  115  to the counter bias control mechanism  104 . In the embodiment, a change of 50 pF or more was sufficient. 
     The cleaning end point determination control like this is feasible using an end point determination circuit  191  illustrated in  FIG. 13 . In other words, in the control parameters monitored in etching or in cleaning, parameters sensitive to the time variation for cleaning are extracted (the monitored parameters in  FIG. 12 , for example), the monitor signal of the counter bias voltage Vpp is calculated as described above based on the signals of the monitored parameters, and the end point of the removal of the attachment on the shower plate is determined. The end point determination circuit  191  may be externally installed on a previously existing device, or can be implemented by changing control software for a previously existing device when monitor signals exist. 
       FIG. 14  is a graph of the oxidization film etching rate in the center part of the shower plate when the RF bias power is changed in resonance (a reference numeral  710  in  FIG. 14 ) and in dissonance (a reference numeral  720  in  FIG. 14 ). In resonance, it is shown that the oxide film rate on the shower plate is almost linearly increased in association with an increase in the RF bias power. Therefore, the method for controlling ion energy on the shower plate can be implemented by resonating the counter bias control mechanism  104  to adjust the RF bias power. Only the RF bias power is adjusted, so that ion energy on the shower plate can be changed without operating the variable capacitor  202  of the counter bias control mechanism  104 , which is advantageous to prolong the lifetime of the variable capacitor. 
     Moreover, the in-plane distribution of the cleaning rate on the shower plate is matched with the plasma distribution, so that control is feasible by resonating the counter bias control mechanism  104  to adjust the electric currents of the electromagnet A  105  and the electromagnet B  106 . It may be possible that with the use of these characteristics, after determining the cleaning end point of the shower plate, the coil current or the RF bias power is changed for over cleaning for a certain time period to improve the efficiency of removal of the attachment on the surface. 
     The example in  FIG. 12  shows the example in which the attachment tends to be removed as fluorocarbon attachments are removed using oxygen. In the case of removing AlF 3 , HfF 4 , and TiO 2 , for example, an ion assist reaction is necessary. In the case of cleaning these compounds, a mixed gas is used in which a gas including reductive H or B (HCl and BCl 3 , for example) that easily breaks coupling such as Al—F and Ti—O is mixed with a gas that increases the volatility of a reaction product with Al, Hf, and Ti in Cl 2 , HBr, and SiCl 4 , for example. The counter bias control mechanism  104  is then resonated to apply an RF bias of 100 W or more to an Si wafer, so that the attachment on the shower plate can be efficiently removed. 
     In the embodiment, the magnetic field parallel plate etching apparatus including a dielectric in the upper electrode as illustrated in  FIG. 8  is described. Similar effects can be obtained in cleaning a parallel plate etching apparatus with no magnetic field as illustrated in  FIG. 15 , in which the circuit constant of a counter bias control mechanism  104  is changed according to Equations (1) to (3) depending on the frequency of a source power supply  101  or an RF bias power supply  123  for use. The control of the cleaning distribution in this case is conducted using the power of the source power supply  101 , the process pressure, and the power of the RF bias power supply  123 . 
     Moreover, as illustrated in  FIG. 15 , the attachment on a silica internal cylinder  804  can be similarly cleaned in which an etching chamber  108  is electrically insulated from a base flange using an insulation ring A  801  and an insulation ring B  803 , an RF bias current passes through a source frequency ground circuit  802  through the silica internal cylinder  804 , and a counter bias control mechanism  104  controls the RF bias current connected to the etching chamber  108 . 
     According to the embodiments as described above, it is possible to provide a plasma processing method that reduces the wearing out of the dielectric ceramic on the upper antenna side as in a CCP etching apparatus, for example, including the counter bias control mechanism and improves MTBM and CoC of the apparatus. 
     As described above, the invention of the present application is described in detail. The following is the main aspects of the present invention.
     (1) A plasma processing method using a plasma processing apparatus including a plasma processing chamber configured to plasma-process an object to be processed, a first flat electrode configured to emit a radio frequency into the plasma processing chamber, a first radio frequency power supply configured to supply radio frequency power to the first electrode, a second electrode opposite to the first electrode and on which the object to be processed is placed, a second radio frequency power supply configured to supply radio frequency power to the second electrode, and a control mechanism configured to control a radio frequency current carried through the first electrode or a radio frequency voltage applied to the first electrode, the method including: a first step of setting a reactance of a variable element included in the control mechanism to an initial value; a second step of detecting the radio frequency current or the radio frequency voltage; a third step of setting the reactance of the variable element to a reactance value so that the radio frequency current takes a maximum value or the radio frequency voltage takes a maximum value and fixing the reactance of the variable element to the set reactance value; and a fourth step of plasma-processing the object to be processed.   (2) A plasma processing method using a plasma processing apparatus including a plasma processing chamber configured to plasma-process an object to be processed, a first flat electrode configured to emit a radio frequency into the plasma processing chamber, a first radio frequency power supply configured to supply radio frequency power to the first electrode, a second electrode opposite to the first electrode and on which the object to be processed is placed, a second radio frequency power supply configured to supply radio frequency power to the second electrode, and a control mechanism configured to control a radio frequency current carried through the first electrode or a radio frequency voltage applied to the first electrode, the method including: a first step of detecting a phase difference between a radio frequency current carried through the second electrode and a radio frequency current carried through the first electrode or a phase difference between a radio frequency voltage applied to the second electrode and a radio frequency voltage applied to the first electrode; a second step of controlling a reactance of a variable element included in the control mechanism so that the detected phase difference takes a phase difference value matched with a maximum value of the radio frequency current carried through the first electrode or a maximum value of the radio frequency voltage applied to the first electrode; and a third step of plasma-processing the object to be processed.   (3) A plasma processing method using a plasma processing apparatus including a plasma processing chamber configured to plasma-process an object to be processed, a first flat electrode configured to emit a radio frequency into the plasma processing chamber, a first radio frequency power supply configured to supply radio frequency power to the first electrode, a second electrode opposite to the first electrode and on which the object to be processed is placed, a second radio frequency power supply configured to supply radio frequency power to the second electrode, and a control mechanism configured to control a radio frequency current carried through the first electrode or a radio frequency voltage applied to the first electrode, the method including: a first step of setting a reactance of a variable element included in the control mechanism to an initial value; a second step of detecting the radio frequency current or the radio frequency voltage; a third step of setting the reactance of the variable element to a reactance value so that the radio frequency current takes a maximum value or the radio frequency voltage takes a maximum value and fixing the reactance of the variable element to the set reactance value; and a fourth step of plasma-cleaning the inside of the plasma processing chamber after the third step.   (4) A plasma processing method using a plasma processing apparatus including a plasma processing chamber configured to plasma-process an object to be processed, a first flat electrode configured to emit a radio frequency into the plasma processing chamber, a first radio frequency power supply configured to supply radio frequency power to the first electrode, a second electrode opposite to the first electrode and on which the object to be processed is placed, a second radio frequency power supply configured to supply radio frequency power to the second electrode, and a control mechanism configured to control a radio frequency current carried through the first electrode or a radio frequency voltage applied to the first electrode, the method including: a first step of detecting a phase difference between a radio frequency current carried through the second electrode and a radio frequency current carried through the first electrode or a phase difference between a radio frequency voltage applied to the second electrode and a radio frequency voltage applied to the first electrode; a second step of controlling a reactance of a variable element included in the control mechanism so that the detected phase difference takes a phase difference value matched with a maximum value of the radio frequency current carried through the first electrode or a maximum value of the radio frequency voltage applied to the first electrode; and a third step of plasma-cleaning the inside of the plasma processing chamber after the second step.   

     It is noted that the present invention is not limited to the foregoing embodiments, and includes various exemplary modifications. For example, the forging embodiments are described in detail for easily understanding the present invention. The present invention is not always limited to ones including all the described configurations. Moreover, a part of the configuration of an embodiment can be replaced by the configuration of another embodiment, and the configuration of another embodiment can be added to the configuration of an embodiment. Furthermore, a part of the configurations of the embodiments can be added with, deleted from, or replaced by the other configurations.