Patent Publication Number: US-11658011-B2

Title: Plasma processing apparatus

Description:
CLAIM FOR PRIORITY 
     This application is a divisional of application Ser. No. 15/091,730, filed on Apr. 6, 2016, which is a continuation of application Ser. No. 14/452,578, filed on Aug. 6, 2014, now U.S. Pat. No. 9,349,603, which is a continuation of application Ser. No. 13/363,415, filed on Feb. 1, 2012, now U.S. Pat. No. 8,828,254, which claims the benefit of Japanese Application No. 2011-163831 filed on Jul. 27, 2011, Japanese patent application No. 2011-211896 filed on Sep. 28, 2011 and Japanese patent application No. 2011-232446 filed on Oct. 24, 2011, in the Japanese Patent Office, the disclosures of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a plasma processing method and a plasma processing apparatus, and particularly to a plasma processing method and a plasma processing apparatus related to plasma etching. 
     Description of the Related Art 
     One of plasma etching apparatuses currently used for the mass production of semiconductor devices is an electron cyclotron resonance (hereinafter referred to as ECR) type apparatus. In this plasma etching apparatus, by applying a magnetic field to a plasma, and setting magnetic field strength so that the frequency of a microwave resonates with the cyclotron frequency of electrons, a high density plasma can be generated. 
     With the miniaturization of semiconductor devices in recent years, the thickness of the gate oxide film has been 2 nm or less. Therefore, it has been necessary to achieve the controllability of plasma etching processing, and a high selection ratio of the gate oxide film to the silicon film. 
     One of techniques for achieving the high precision plasma etching is a plasma etching method using pulsed discharge. For example, Japanese Patent Laid-Open Publication No. 09-185999 (Patent Document 1) discloses a method for attaining high precision etching by pulse-modulating a plasma, while measuring radical density in the plasma, to control the radical density. 
     In addition, Japanese Patent Laid-Open Publication No. 09-092645 (Patent Document 2) discloses a method for pulse-modulating a plasma and simultaneously synchronizing the phase of a radio-frequency bias applied to a wafer with the ON-OFF of the plasma to control the temperature of electrons in the plasma to prevent the dielectric breakdown of an oxide film on the wafer being processed. 
     In addition, Japanese Patent Laid-Open Publication No. 08-181125 (Patent Document 3) discloses a method for preventing the dielectric breakdown of an oxide film and simultaneously attaining high rate anisotropic etching by pulse-modulating a plasma at 10 to 100 μs and applying a radio-frequency bias at 600 KHz or less to a wafer. 
     In addition, Japanese Patent Laid-Open Publication No. 06-267900 (Patent Document 4) discloses a method for pulse-modulating a microwave for generating a plasma, to control radicals, and suppressing the instability of the plasma to decrease ion temperature. 
     Generally, an ECR type plasma etching apparatus has three problems as shown below. 
     The first problem is that there is a case where a lower density region (lower microwave power) is required according to processing, such as for an improvement in perpendicularity, but when the microwave power is decreased in order to decrease the plasma density, the production of a plasma is difficult. 
     The second problem is that when a discharge test is performed changing the power of the microwave, there is an unstable region in which the light emission of the plasma is seen as flickering in visual inspection or measurement by a photodiode or the like, depending on the microwave power. There is no reproducibility of characteristics, such as an etching rate, in this region, and therefore, etching conditions are set avoiding the unstable region, that is, a narrow process window is set in process development. 
     The discharge flicker of interest in the present invention is a phenomenon in which the electric field strength distribution within the chamber changes, depending on the microwave power, abnormal discharge is generated, for example, near the sample stage or near the microwave transmission window, in connection with the chamber shape, and blinking can be observed by visual inspection. 
     The third problem is that there is a high selection ratio region on the high microwave power side (high density region), but on the high microwave power side, a cut-off phenomenon occurs with an increase in plasma density, and a mode jump in which the distribution of plasma density in the chamber changes occurs. When this phenomenon occurs, the light emission intensity of the plasma and the peak value of the bias voltage (Vpp voltage) change suddenly, and with this, the distribution of the etching rate in the wafer surface also changes largely, and therefore, power around the mode jump can not be used. 
     These three problems are also common to an ECR type plasma etching apparatus using pulsed discharge. But, in the above-described conventional techniques, these three problems are not considered. 
     Therefore, the present invention provides a plasma processing method and a plasma processing apparatus in which a stable process region can be ensured in a wide range, from low microwave power to high microwave power. 
     SUMMARY OF THE INVENTION 
     The present invention is a plasma processing method for facilitating production of a plasma in a region in which production of a plasma by continuous discharge is difficult, and plasma-processing an object to be processed with such generated plasma, the method including: using pulsed discharge in which ON and OFF are repeated to facilitate production of the plasma, wherein radio-frequency power for producing the pulsed discharge, during an ON period, is such a power as to facilitate the production of a plasma by the continuous discharge, and a duty ratio of the pulsed discharge is controlled so that an average power of the radio-frequency power per cycle matches power in the region in which the production of a plasma by the continuous discharge is difficult. 
     In addition, the present invention is a plasma processing apparatus including a processing chamber in which a plasma is produced, plasma production means for producing the plasma, and a sample stage, on which a wafer is placed, provided within the processing chamber, and etching the wafer with the plasma, wherein the plasma production means includes a power supply for supplying power for producing the plasma, and a time-average value of the power is controlled by ON-OFF-modulating the power of the power supply, setting peak power during ON to a value at which when a plasma is generated by continuous discharge, instability of the plasma does not occur, and changing a duty ratio of the ON-OFF modulation. 
     In addition, the present invention is a plasma processing apparatus including a chamber which can be evacuated to introduce a reactive gas, a plasma production power supply for producing a discharge plasma within the chamber, and a sample stage, on which a wafer is mounted, within the chamber, further including means for pulse-modulating an output power of the plasma production power supply, setting peak power during ON to a power value sufficiently higher than a mode jump region in continuous discharge, and changing a duty ratio of the pulse modulation to control a time-average value of the power. 
     According to the present invention, a stable process region can be ensured in a wide range, from low microwave power to high microwave power. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic cross-sectional view showing one example of a plasma etching apparatus for carrying out the etching method of the present invention; 
         FIG.  2    shows the output waveform of the magnetron  106  of the apparatus shown in  FIG.  1   ; 
         FIG.  3    is a graph showing the effect of improving a selection ratio according to the present invention; 
         FIG.  4    is a graph showing the dependence of the light emission intensity of oxygen atoms on the time-average output of a microwave according to the present invention; 
         FIG.  5    is a schematic cross-sectional view of a plasma etching apparatus which performs feedback control in which the duty ratio of pulsed discharge is changed for each wafer according to the Exemplary Embodiment 4 of the present invention; 
         FIG.  6    is a cross-sectional view of a fine pattern on a wafer to be processed; 
         FIG.  7    is a diagram showing the data of the correlation between microwave power (duty ratio) and CD in the Exemplary Embodiment 4 of the present invention; 
         FIG.  8    is a schematic cross-sectional view showing a plasma etching apparatus in the Exemplary Embodiment 5 of the present invention; 
         FIG.  9    is a graph showing an O (oxygen)/Br (bromine) light emission intensity ratio with respect to a microwave according to the present invention; 
         FIG.  10    is a graph showing a polysilicon/silicon oxide film selection ratio with respect to a pulsed microwave according to the present invention; and 
         FIG.  11    is a process flow chart of an apparatus for automating the measurement of power in which a mode jump occurs. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First, one example of a plasma etching apparatus for carrying out the present invention will be described with reference to the drawings.  FIG.  1    is a schematic cross-sectional view of a microwave ECR plasma etching apparatus using a microwave and a magnetic field for plasma production means. 
     The microwave ECR plasma etching apparatus includes a chamber  101  which can be evacuated, a sample stage  103  on which a wafer  102 , an object to be processed, is disposed, a microwave transmission window  104 , such as quartz, provided on the upper surface of the chamber  101 , a waveguide  105  and a magnetron  106  provided above the microwave transmission window  104 , a solenoid coil  107  provided around the chamber  101 , and an electrostatic adsorption power supply  108  and a radio-frequency power supply  109  connected to the sample stage  103 . 
     The wafer  102  is carried into the chamber  101  from a wafer carry-in port  110 , and then electrostatically adsorbed on the sample stage  103  by the electrostatic adsorption power supply  108 . Next, a process gas is introduced into the chamber  101 . The chamber  101  is evacuated to reduce pressure by a vacuum pump (not shown), and adjusted to a predetermined pressure (for example, 0.1 Pa to 50 Pa). Next, a microwave having a frequency of 2.45 GHz is oscillated from the magnetron  106 , and propagated into the chamber  101  through the waveguide  105 . The process gas is excited by the action of the microwave and a magnetic field generated by the solenoid coil  107 , and a plasma  111  is formed in a space above the wafer  102 . On the other hand, a bias is applied to the sample stage  103  by the radio-frequency power supply  109 , and ions in the plasma  111  are perpendicularly accelerated onto the wafer  102  and enter the wafer  102 . In addition, the radio-frequency power supply  109  can apply continuous radio-frequency power or time-modulated intermittent radio-frequency power to the sample stage  103 . 
     The wafer  102  is anisotropically etched by the action of radicals and ions from the plasma  111 . In addition, a pulse generator  112  is attached to the magnetron  106 , and thus, the microwave can be pulse-modulated in a pulse form at a repetition frequency which can be optionally set, as shown in  FIG.  2   . 
     In addition, when the microwave ECR plasma etching apparatus plasma-processes the wafer  102 , a control part  113  controls each of the magnetron  106 , the pulse generator  112 , the solenoid coil  107 , the radio-frequency power supply  109 , and the electrostatic adsorption power supply  108  described above. 
     The microwave ECR plasma etching apparatus according to the present invention, used in Exemplary Embodiments described below, is a microwave ECR plasma etching apparatus which processes a wafer having a diameter of 300 mm. The inner diameter of the chamber  101  is 44.2 cm, and the distance between the wafer  102  and the microwave transmission window  104  is 24.3 cm (the volume of the space in which the plasma is generated: 37267 cm 3 ). 
     Next, the present invention for solving the first problem will be described by Exemplary Embodiment 1 and Exemplary Embodiment 2. The first problem is that when the microwave power is decreased in order to decrease the plasma density, the production of a plasma is difficult. 
     Exemplary Embodiment 1 
     The ignition characteristics of a plasma, depending on the duty ratio, and the time-average output of the microwave, were examined under conditions shown in Table 1, using the microwave ECR plasma etching apparatus shown in  FIG.  1   . Vpp in Table 1 is the voltage difference between a peak value and a peak value of a radio-frequency voltage applied to the sample stage  103 . 
     In addition, the results of examining plasma production are shown in Table 2. “Good” in Table 2 indicates that a plasma was stably produced, and “Poor” indicates that a plasma could not be stably produced. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
             
            
               
                   
                 HBr gas 
                 200  
                 ml/min 
               
               
                   
                 O 2  gas 
                 3  
                 ml/min 
               
               
                   
                 Ar gas 
                 200  
                 ml/min 
               
               
                   
                 Ar + CH 4  (4%) gas 
                 30  
                 ml/min 
               
               
                   
                 Process pressure 
                 0.8  
                 Pa 
               
            
           
           
               
               
               
            
               
                   
                 RF bias 
                 Vpp 350 V constant (about 
               
               
                   
                   
                 40 W) 
               
            
           
           
               
               
               
               
            
               
                   
                 Microwave ON-OFF repetition 
                 5  
                 kHz 
               
            
           
           
               
               
               
            
               
                   
                 frequency 
                   
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                   
                 Microwave time-average output (W/cm 3 )  
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 0.0027 
                 0.0054 
                 0.007 
                 0.0086 
                 0.011 
               
               
                 Duty 
                 (about 
                 (about 
                 (about 
                 (about 
                 (about 
               
               
                 ratio 
                 100 W) 
                 200 W) 
                 260 W) 
                 320 W) 
                 400 W) 
               
               
                   
               
               
                  20% 
                 Good 
                 Good 
                 Good 
                 Good 
                 Good 
               
               
                  50% 
                 Poor 
                 Good 
                 Good 
                 Good 
                 Good 
               
               
                  65% 
                 Poor 
                 Poor 
                 Good 
                 Good 
                 Good 
               
               
                  80% 
                 Poor 
                 Poor 
                 Poor 
                 Good 
                 Good 
               
               
                 100% 
                 Poor 
                 Poor 
                 Poor 
                 Poor 
                 Good 
               
               
                 (Continuous 
                   
                   
                   
                   
                   
               
               
                 discharge) 
               
               
                   
               
            
           
         
       
     
     From Table 2, in the case of continuous discharge, a plasma is stably produced when a value obtained by dividing the time-average output of the microwave by the volume of the inner wall of the chamber  101  is 0.011 W/cm 3  (about 400 W) or more, and a plasma is not produced when the value is less than 0.011 W/cm 3 . The reason why a plasma is not produced is that the energy required for free electrons to ionize the molecules of the etching gas is not supplied. But, in the cases of discharge with a pulsed microwave, a plasma is produced even if a value obtained by dividing the time-average output of the microwave by a volume in which the plasma is produced is smaller than 0.011 W/cm 3 . 
     This difference in plasma production between by the continuous discharge and by the pulsed microwave discharge is due to the following reason. 
     During several μsec during a period during which the microwave is ON, free electrons ionize or dissociate other atoms and molecules with energy obtained from the microwave, to produce the plasma  111 . Then, during a period during which the microwave is OFF, most of the free electrons are captured by the atoms and the molecules during several μsec, and a large portion of the plasma  111  forms anions and cations. Since the anions and the cations have a larger mass than the electrons, the anions and the cations collide with each other and are neutralized, and a time of several tens of ms is required until the plasma  111  dissipates. Therefore, when the OFF time of the microwave is shorter than 10 ms, the ON period of the microwave starts before the plasma  111  dissipates, and the plasma  111  is maintained. 
     Therefore, when the results shown in Table 2 are put another way, it can be said that when the output during the ON period of the pulsed microwave is equal to or more than the minimum output required to stably produce a plasma by continuous discharge (that is, 0.011 W/cm 3  or more), a plasma can be stably produced even if the time-average output of the pulsed microwave is 0.011 W/cm 3  or less. Further, it can be said that when the OFF period of the pulsed microwave is 10 ms or less, a plasma can be more stably produced than the above-described plasma processing method, even in a region in which the time-average output of the pulsed microwave is 0.011 W/cm 3  or less. 
     Further, a plasma is easily stably produced by adding an inert gas, such as an argon gas (Ar, ionization energy: 1520.6 kJ/mol) or a nitrogen gas (N 2 , ionization energy: 1402 kJ/mol), or adding an easily ionizable gas other than the above inert gas. The type of other gases, the flow rate of each gas, the process pressure, the RF bias value, and the like do not specially limit the effect of this Exemplary Embodiment. 
     Next, a wafer having a polysilicon film on the entire surface thereof and a wafer having a silicon oxide film on the entire surface thereof were etched under the conditions in Table 1, and a selection ratio was obtained from the ratio of their respective amounts of etching. The results are shown in  FIG.  3   . “%” in  FIG.  3    means a duty ratio when pulsed discharge was performed. 
     From  FIG.  3   , it is seen that when the duty ratio is 50% or less, and the microwave time-average output is 400 W (0.011 W/cm 3 ) or less, the selection ratio increases more than in continuous discharge. The reason for this is considered to be that when the time-average output of the microwave is small, the number of free electrons within the chamber  101  decreases, and electrons corresponding to this decrease excite atoms and molecules, and therefore, the density of the radical species increases, and the selection ratio is improved. 
     In addition, the selection ratio is further increased by controlling the oxygen gas in the conditions in Table 1 at 1 ml/min or more and 10 ml/min or less. 
     In addition, while this Exemplary Embodiment is an example in which the plasma processing method of the present invention is applied to the microwave ECR plasma etching apparatus, the plasma processing method of the present invention can be similarly applied to a capacitively coupled or inductively coupled plasma etching apparatus. As described above, the plasma processing method of the present invention is a method for processing an object to be processed by pulsed discharge in which the radio-frequency power during the ON period of the pulsed discharge is radio-frequency power in which a plasma by continuous discharge can be stably produced, and the OFF period of the pulsed discharge is 10 ms or less. By such a plasma processing method of the present invention, a plasma can be stably produced even in a region in which the plasma production power is small, in which it is difficult to stably produce a plasma. 
     Further, in the plasma processing method of the present invention, when the duty ratio of the pulsed discharge is 50% or less, the selection ratio of the etching rate of the polysilicon film to the silicon oxide film can be more improved than in continuous discharge. 
     Exemplary Embodiment 2 
     Next, another Exemplary Embodiment of the present invention described in Exemplary Embodiment 1 will be described. 
     When plasma processing by pulsed discharge is performed under conditions shown in Table 3, the performance of the removal of the carbon-based deposits and the removal of the resist is more improved than in continuous discharge. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
             
            
               
                   
                 Oxygen (O 2 ) gas 
                 200  
                 ml/min 
               
               
                   
                 Process pressure 
                 1.0  
                 Pa 
               
               
                   
                 RF bias 
                 0  
                 W 
               
               
                   
                 Microwave ON-OFF repetition 
                 5  
                 kHz 
               
            
           
           
               
               
               
            
               
                   
                 frequency 
                   
               
               
                   
                   
               
            
           
         
       
     
     The ratio of the light emission intensity of oxygen to the light emission intensity of argon was measured, adding an argon gas at 5 ml/min under the conditions shown in Table 3. The results are shown in  FIG.  4   . 
     From  FIG.  4   , it is seen that the light emission intensity of oxygen atoms is strong at less than 0.011 W/cm 3 , a value obtained by dividing the time-average output of the microwave by a volume in which the plasma is generated. 
     In other words, when the pulsed discharge of the present invention is performed under the conditions shown in Table 3, the oxygen radical density increases, and the effect of the removal of the carbon-based deposits and the removal of the resist is improved. 
     Next, the present invention for solving the second problem will be described by Exemplary Embodiment 3 to Exemplary Embodiment 5. The second problem is that when a discharge test is performed changing the power of the microwave, there is an unstable region in which the light emission of the plasma is seen as flickering in visual inspection or measurement by a photodiode or the like, depending on the microwave power. 
     Exemplary Embodiment 3 
     The plasma etching in this Exemplary Embodiment used the microwave ECR plasma etching apparatus shown in  FIG.  1   . Next, an example of conditions for etching a polysilicon film  302  is shown in Table 4. Under the conditions, the polysilicon film  302  can be etched at a high selection ratio to a base oxide film  303 . 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 4 
               
               
                   
                   
               
             
            
               
                   
                 HBr gas 
                 140  
                 ml/min 
               
               
                   
                 O 2  gas 
                 2  
                 ml/min 
               
               
                   
                 Process pressure 
                 1.6  
                 Pa 
               
            
           
           
               
               
               
            
               
                   
                 RF bias (output per wafer 
                 50 W (0.071 W/cm 2 ) 
               
               
                   
                 area) 
                   
               
               
                   
                   
               
            
           
         
       
     
     Changing the microwave for generating a plasma, under the conditions shown in Table 4, light emission from the plasma  111  was detected by a photodiode, and its flicker was measured. The results are shown in Table 5. For the power of the microwave, a case where continuous discharge was performed is compared with a case where the power was controlled by setting the power during the ON period of the microwave to 1500 W, performing ON-OFF modulation at a repetition frequency of 1 KHz, and changing the duty ratio. In Table 5, “Good” indicates that there was no discharge flicker, and “Poor” indicates that there was discharge flicker. Etching cannot be performed in a state in which discharge flickers. 
     
       
         
           
               
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                   
                 Time-average microwave power (W) 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 400 
                 500 
                 600 
                 700 
                 800 
                 900 
                 1000 
                 1100 
                 1200 
                 1300 
                 1400 
                 1500 
               
               
                   
               
               
                 Continuous 
                 Good 
                 Good 
                 Good 
                 Good 
                 Good 
                 Poor 
                 Poor 
                 Poor 
                 Good 
                 Good 
                 Good 
                 Good 
               
               
                 discharge 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Pulsed 
                 Good 
                 Good 
                 Good 
                 Good 
                 Good 
                 Good 
                 Good 
                 Good 
                 Good 
                 Good 
                 Good 
                 Good 
               
               
                 discharge 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 (peak power: 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 1500 W) 
               
               
                   
               
            
           
         
       
     
     In the continuous discharge, flicker occurs at 900 W to 1100 W, but the flicker of the discharge can be eliminated by the ON-OFF control of the microwave. The reason is presumed to be that the plasma  111  generated by instantaneous power is set to be in a stable region, and further, when pulsed discharge is performed with the plasma  111  of a gas that easily forms negative ions, such as a halogen gas, electrons disappear in several tens of μs during OFF, and for several ms after this, negative ions and positive ions are involved in maintaining the discharge, and therefore, the state of the sheath of the plasma  111  produced at the interface between the chamber wall and the plasma  111  is different from that in continuous discharge, and the flicker is eliminated. 
     The time until the plasma  111  dissipates is several tens of ms, and therefore, when the OFF time is 10 ms or less, ON starts before the plasma  111  dissipates, and the plasma is maintained. 
     The power region in which the plasma  111  flickers depends on the conditions. Therefore, in etching under other conditions, flicker can be eliminated when, first, the microwave power is changed in continuous discharge to check a region in which the discharge flickers, as in Table 5, to set the power during the ON period of the microwave sufficiently larger than power in which flicker occurs, and the microwave is ON-OFF pulse-modulated at a frequency at which the OFF time is 10 ms or less. 
     When the size of the chamber  101  changes, the power of the microwave shown in Table 5 changes according to its volume. 1500 W corresponds to about 0.04 W/cm 3  when converted to microwave power per unit volume. 
     The fact that there is an unstable region in discharge is not limited to microwave plasma etching apparatuses. Inductively coupled or capacitively coupled plasma etching apparatuses also have a similar problem. Also in these apparatuses, discharge instability can be avoided by the present invention. 
     Exemplary Embodiment 4 
     Next, an Exemplary Embodiment concerning a method for controlling an etching processing dimension (hereinafter referred to as “CD”) enabled by the ON-OFF modulation of the plasma  111  will be described.  FIG.  5    shows a schematic view of a plasma etching apparatus in which a mechanism for measuring an etching end time obtained from the light emission intensity of the plasma  111  or a change in light emission intensity, or the like, and, based on this monitored value, changing the etching conditions of the wafer  102  being processed or the wafer  102  to be processed next is added to the microwave ECR plasma etching apparatus shown in  FIG.  1   . 
     A light receiving part  202 , a CD calculation part  203 , a recipe calculation part  205 , a first database  206 , a second database  204 , and an etching control PC  207  shown in  FIG.  5    are communicatively coupled via communication means.  FIG.  6    is a cross-sectional view of a fine pattern on the wafer  102  to be processed and shows a state in which a polysilicon film  302  lying on a silicon substrate  304  and a base oxide film  303  is etched in the form of the same pattern as a mask  301 , such as silicon nitride, processed in the form of a fine pattern. 
     In dry etching, usually, processing as shown in  FIG.  6    is continuously performed for 1 lot (25 wafers). The processed line width (hereinafter referred to as “CD”) needs to be within a certain tolerance during the continuous processing. But, there is a case where reaction products of etching, and the like adhere to the inside of the chamber  101 , and so on, and thus, the plasma state changes with time, and the fluctuations in CD are not within a tolerance. 
     In this Exemplary Embodiment, the fluctuations in CD are controlled within a tolerance by ON-OFF-modulating the plasma  111  and changing the duty ratio for each wafer. Usually, the CD changes, depending on the bias power applied to the wafer  102 , and the plasma density, that is, the microwave power, and therefore, the CD can be changed by changing the microwave power. 
     Next, a specific method will be described. The end point of the etching of the polysilicon film  302  shown in  FIG.  6    is detected by the light emission of a reaction product in the plasma  111 , for example, light of silicon having 426 nm, by an optical fibre  201  and the light receiving part  202 . There is a correlation between the end time of etching and CD, and the relationship between the etching end time and CD is stored in the second database  204 . The CD calculation part  203  calculates an estimate of the CD of this wafer  102  from the etching end time. The difference between the calculated CD and a CD target value is calculated, and the value of this difference is sent to the recipe calculation part  205 . 
     The recipe calculation part  205  has the first database  206  in which the data of the correlation between microwave power (duty ratio) and CD shown in  FIG.  7    is stored, and calculates the variable of microwave power required to make the difference from the target value of CD zero. For example, as shown in  FIG.  7   , when the target CD is 30 nm, and the CD of the n-th wafer is 30+a (nm), the average microwave power, that is, the duty ratio, for the n+1-th wafer is increased by d (%) in order to achieve the target CD, that is, in order to narrow CD by a (nm). 
     The duty ratio calculated from the first database  206  is sent to the etching control PC  207 . When the next wafer  102  is processed, etching is performed setting this value. At this time, when the plasma  111  is continuously discharged, the microwave power value corrected so that the CD difference is zero may be in the unstable region of the plasma  111  shown in Table 5, which hinders etching. As described in Exemplary Embodiment 3, when the microwave power is controlled by ON-OFF-modulating the plasma  111  and changing the duty ratio, the problem of the instability of the plasma  111  can be eliminated. 
     Exemplary Embodiment 5 
     Next, a method that increases the margin of stability more when used in combination with the present invention in order to prevent discharge instability will be described by  FIG.  8   . First, in order to stabilize the potential of the plasma  111 , it is desired to provide an earth surface  401  through which a direct current flows, in a portion to be in contact with the plasma  111 . 
     Usually, the inner wall of the chamber  101  is subjected to stabilization with alumite, yttrium oxide, or the like. These materials are insulators, and therefore, a direct current does not flow through them. By peeling these insulating films, inserting a conductor, or the like, in part of the portion to be in contact with the plasma  111 , and further setting the conductor portion at an earth potential, the potential of the plasma  111  is stabilized, and therefore, the discharge is more stabilized. It is desired that the area of the direct current earth surface  401  is 10 cm 2  or more. 
     Next, it is desired that the pressure of the process gas is set to 0.1 to 10 Pa. If the pressure is too low, the mean free path of electrons increases, and a chance that the electrons dissipate on the wall before ionization occurs increases, which causes the instability of the plasma  111 . In addition, if the pressure is too high, the ignition characteristics worsen, and instability occurs easily. 
     Further, for the shape of the chamber  101  and the sample stage  103 , it is desired to decrease portions in which the electric field is locally strong to a minimum. In other words, sharp unevenness should not be provided, and a corner portion  402  should have a curve having a radius of 5 mm or more. 
     Next, the present invention for solving the third problem will be described by Exemplary Embodiment 6 and Exemplary Embodiment 7. The third problem is that there is a high selection ratio region on the high microwave power side (high density region), but on the high microwave power side, a cut-off phenomenon occurs with an increase in plasma density, and a mode jump in which the distribution of plasma density in the chamber changes occurs. 
     Plasma etching in Exemplary Embodiment 6 and Exemplary Embodiment 7 was performed using the microwave ECR plasma etching apparatus shown in  FIG.  1   . 
     Exemplary Embodiment 6 
     As shown in  FIG.  3   , the polysilicon-to-silicon oxide film selection ratio also increases in a region in which the average microwave power is large (800 W or more). In order to explain the reason for this, the relationship between microwave power and the light emission intensity ratio of O (oxygen) to Br (bromine) is shown in  FIG.  9   . As is seen from  FIG.  9   , the light emission intensity of O (oxygen), that is, the density of O radicals, increases on the high microwave side. Therefore, it is considered that the etching of the silicon oxide film is suppressed, and the selection ratio is improved. 
     However, when the microwave power value is further increased, a sudden change in light emission intensity, that is, the above mode jump phenomenon (during CW) 500, occurs at a microwave power value of 900 W or more in CW (continuous discharge), as shown in  FIG.  9   . Therefore, the high microwave region can not be used in continuous discharge. 
     Therefore, in the present invention, the microwave is pulsed, the peak power during ON is set sufficiently higher than the power value at which the mode jump occurs, and the duty ratio is controlled. It is seen that at a duty of 65% or less, there is no sudden change in light emission intensity ratio ( FIG.  9   ), that is, a mode jump is avoided. 
     The reason for this is presumed as follows. In the CW (continuous discharge), the electron density increases with the microwave power value, and the mode changes from a power value at which a density at which the oscillation of the plasma  111  resonates with the frequency of the electromagnetic wave is reached. On the other hand, with the pulsed microwave, during OFF, most of free electrons are captured by atoms and molecules during several μsec, and a large portion of the plasma  111  forms anions and cations. Therefore, with the pulsed microwave in which ON-OFF is repeated, an increase in electron density does not occur. 
     The results of the selection ratio evaluated on the high microwave power side, using the conditions in Table 1, are shown in  FIG.  10   . In the CW (continuous discharge), the selection ratio decreases (becomes unstable) at a microwave power value of 900 W or more due to the effect of the mode jump, whereas by pulsing the microwave, the high microwave power side can be used, and a high selection ratio can be obtained. 
     It is known that, generally, in pulsed discharge, the electron density decreases by one or more orders of magnitude in 50 μs after the power is turned OFF. Therefore, when discharge is pulsed, and the repetition frequency of the pulse and the duty ratio are set so that the OFF time of the pulsed discharge is 50 μs or more, a mode jump can be sufficiently avoided. 
     As described above, in the present invention, by pulse-modulating the microwave, a mode jump which occurs on the high microwave power side can be avoided, and the process region effective for etching can be expanded. 
     Exemplary Embodiment 7 
     Next, a method and an apparatus for automatically avoiding this mode jump region will be described. A mode jump occurs in a high power region of a microwave power of about 900 W or more, as shown in  FIG.  9   . The density of the plasma  111  is different depending on the pressure of the gas and the type of the gas, and therefore, the power in which a mode jump occurs is also different depending on these conditions. 
     In the method for avoiding this, first, a power value at which a mode jump occurs under the conditions used is previously measured, and the apparatus is adapted to include the function of storing etching conditions and power in which a mode jump occurs under the conditions, and automatically pulse-modulating microwave power when using the power in which a mode jump occurs under the conditions. In an apparatus having this function, the erroneous operation of erroneously using a mode jump region can be prevented. 
     Further, an apparatus for previously automating the measurement of power in which a mode jump occurs will be described. A process flow chart of an apparatus for automating the measurement of power in which a mode jump occurs is shown in  FIG.  11   . Usually, etching is performed in a lot (25 wafers) unit. Before a lot is processed, dummy wafer processing  501  is performed under the same conditions as the subsequent processing conditions. 
     Then, lot processing  502 , and the cleaning  503  of the chamber  101  with the plasma  111  of oxygen, or the like follow. The dummy wafer processing  501  includes microwave power automatic scanning measurement  504  which is the step of automatically scanning microwave power set values, for example, 800 W to 1200 W, and measuring the light emission intensity of the plasma  111  during this period by a photodiode or the like. 
     Next, an etching apparatus-controlling personal computer  507  includes the function of performing mode jump power identification  505  in which from this data, a region in which the light emission intensity changes suddenly is extracted and stored, and further performing automatic recipe generation  506  in which pulse modulation is automatically performed when the microwave power corresponds to the mode jump region when a recipe is input. Due to this function, a worker can perform etching without being troubled by the mode jump region. 
     In addition, the physical amount that is measured when the microwave power is scanned is not limited to light emission intensity. An amount that changes suddenly according to a mode jump, such as the peak value of a bias voltage (Vpp), can serve the same function. In addition, the absolute value of the microwave power described in the present invention changes largely, mainly according to the size of the chamber  101 , that is, the diameter of the wafer  102  to be processed. As a standard, when a value normalized by the volume of the chamber  101  is used, the microwave power can be converted to an amount that does not depend on the volume of the chamber  101 . For example, 900 W corresponds to 0.024 W/cm 3  in the above Exemplary Embodiments.