Abstract:
A plasma monitoring method using a sensor, the sensor having a substrate; a first electrode, the first electrode being a conductive electrode and formed on the substrate while being isolated from the substrate; an insulating film formed on the first electrode; a contact hole formed in the insulating film and having a depth from a surface of the insulating film to the first electrode; and a second electrode, the second electrode being a conductive electrode, formed on the surface of the insulating film, and faced to plasma during a plasma process, the plasma monitoring method including measuring and monitoring potentials of the first electrode and the second electrode or a potential difference between the first electrode and the second electrode during the plasma process is disclosed. A plasma monitoring system carrying out the plasma monitoring method is also disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a divisional of application Ser. No. 12/219,123, filed on Jul. 16, 2008. Furthermore, this application claims priority under 35 USC 119 from Japanese Patent Application No. 2007-225677, filed on Aug. 31, 2007, the disclosures of which are incorporated by reference herein. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a plasma monitoring method applicable to a semiconductor manufacturing processes (steps) and all the other manufacturing processes using plasma and a plasma monitoring system therefor. 
         [0004]    2. Description of the Related Art 
         [0005]    There is a conventional technique related to a plasma monitoring method and a plasma monitoring system for monitoring a processing on a wafer disposed in a plasma processing apparatus as disclosed in, for example, Japanese Patent Application Laid-Open (JP-A) Nos. 2003-282546 and 2005-236199. 
         [0006]      FIG. 7  is a schematic configuration diagram showing a conventional plasma monitoring system disclosed in the JP-A Nos. 2003-282546 and 2005-236199. 
         [0007]    The plasma monitoring system includes a plasma processing apparatus  10 . The plasma processing apparatus  10  is an apparatus applying a radio frequency (hereinafter, “RF”) bias to a plasma chamber  11  set in a vacuum to generate plasma  12  within the plasma chamber  11 , and performing such processings as etching and film formation on a wafer  20  that is a monitoring target workpiece disposed on a stage  13 . A voltmeter  15  for self-alignment bias measurement is connected to the stage  13  via a coil  14  for alternating current (hereinafter, “AC”) voltage component elimination. A sensor  21  or the like for plasma process detection is bonded onto the wafer  20 . 
         [0008]    If a plasma process is to be monitored, then the plasma  12  is generated in the plasma chamber  11  by application of the RF bias to the plasma chamber  11 , and the plasma process (e.g., plasma etching) is performed on the wafer  20 . At this time, by monitoring a voltage detected by the sensor  21 , a plasma etching end point may be detected and the wafer  20  may be worked with high accuracy. 
         [0009]    It is generally known that energy of ions generated from the plasma  12  during the plasma etching influences a shape and a size of a pattern of the wafer  20  and electrically damages the wafer  20 . Due to this, it is important to observe energy of ions incident on the wafer  20  from the plasma  12  and an ion energy distribution. However, since the ion incident energy if ions may not be directly measured, a self-alignment bias is monitored and set as an indirect index. Normally, the voltmeter  15  disposed below the stage  13  within the plasma chamber  11  measures an average value of the self-alignment bias. Since the self-alignment bias is an AC voltage, the coil  14  eliminates RF component in the AC voltage so that the voltmeter  15  may measure only a constant direct-current (hereinafter, “DC”) voltage. 
         [0010]      FIG. 8  is a schematic diagram explaining the self-alignment bias. As shown in a state 1, when the wafer  20  is exposed to the plasma  12 , the plasma  12  is in a state in which electrons e and positive ions h are slightly separated. Both the electrons e and the positive ions h move to be charged onto the wafer  20 . However, at this time, the electrons e more faster and a large quantity of electrons e are charged onto the wafer  20  (and onto the stage  13  if the stage  13  is present under the wafer  20 ) since the electrons e are far lighter than the positive ions h. Due to this, as shown in a state 2, a potential of the wafer  20  turns negative by the charging of the electrons e on the wafer  20 . 
         [0011]    As shown in a state 3, the positive ions h which are oppositely charged to electrons e, and which move faster than electrons, arrive at the wafer  20 . However, the amount of the positive ions h is not so large as to cancel the electrons e previously charged at the wafer  20 . Due to this, ultimately both the negative electrons e and the positive ions h from the plasma  12  arrive at the wafer  20  and are charged thereat. However, since a charge amount of the initial negative electrons e (in the state 1) is larger, the potential of the wafer  20  is negative in a stable state. This negative potential is referred to as self-alignment bias. 
         [0012]    Nevertheless, the conventional plasma monitoring methods and plasma monitoring systems have a first problem (1) and a second problem (2) as follows. 
       (1) First Problem 
       [0013]    In a working process of forming a large scale integrated circuit (hereinafter, “LSI”) on the wafer  20 , plural contact holes is formed, for example, by plasma etching. However, both a potential of a surface of the wafer  20  and that of a bottom of each contact hole may not be monitored in the conventional technique. Due to this, charge offset caused by trapping of charges (charge-up) may not be measured. If an aspect ratio (a ratio of a depth of each contact hole to a diameter thereof) is high, it is difficult for the electrons e to arrive at bottoms of the contact holes (electron blocking effect). As a result, the electrons e are insufficiently supplied to the bottoms of the contact holes, thereby making the bottoms of the contact holes positively charged up as compared with a surface of a contact hole pattern. These respects provoke such problems as dielectric breakdown of transistors, reduction in etch rate, and insufficient progress of etching. The charge-up problem is serious since the diameter of each contact hole in and after the advanced 65-nanometer (nm) generation is 0.1 micrometer (μm) and the aspect ratio of the contact hole is as high as about 10. 
         [0014]    Generally, a recording memory transistor (Non-Volatile Memory Transistors (hereinafter “NVM”)) or a wafer (blank wafer), on which no circuit pattern is formed, is employed to monitor a charge-up amount. However, even with use of the NVM or the blank wafer, neither the measurement of a charge-up amount on an actual pattern nor that of a charge-up amount at real time may not be advantageously made. A problem related to the NVM (hereinafter, “(a) NVM-related Problem”) and a problem related to the blank wafer (hereinafter, “(b) blank wafer-related Problem”) will be described in detail. 
       (a) NVM-Related Problem 
       [0015]    In case of the NVM, an antenna (a conductor) on the surface of a wafer  20  exposed to the plasma  12  is connected to a gate electrode of the NVM buried in the wafer  20 . A transistor characteristics (easiness of current flow between a source electrode and a drain electrode) of the NVM changes according to a magnitude of a potential applied to the gate electrode of the NVM. Due to this, if charge-up occurs on the NVM charge-up monitoring wafer  20 , charges are trapped into the antenna and a potential of the antenna changes. Since the antenna is connected to the gate electrode of the NVM, the characteristic of the NVM changes according to a potential of the antenna. Namely, an amount of a change in the transistor characteristics may be recognized from a magnitude of a charge-up amount (a potential change width). Therefore, if the charge-up occurs on the NVM charge-up monitoring wafer  20 , then charges are trapped into the antenna and the antenna potential changes. Since the antenna is connected to the gate electrode of each NVM, the NVM characteristic changes according to the magnitude of the antenna potential. Namely, the magnitude of the charge-up amount (potential change width) may be confirmed from the change amount of the transistor characteristics. Accordingly, in case of the NVM, the sensor wafer  20  that is the monitoring target workpiece is temporarily exposed to the plasma  12  to change the NVM characteristic, the sensor wafer is taken out from the plasma  12 , and how much the NVM characteristic changes (a change amount of the easiness for current flow across the NVM) before and after the exposure to the plasma  12  is measured using a measuring instrument. 
         [0016]    Therefore, if the charge-up occurs in the atmosphere of the plasma  12 , the charge-up (e.g., antenna potential) may not be observed at real time. Further, since the antenna (conductor) is flat and the flat antenna (conductor) receives (picks up) the charge-up, the charge-up that occurs in a pattern of an actual LSI product such as contact hole may not be measured. 
       (b) Blank Wafer-Related Problem 
       [0017]    The blank wafer means a wafer configured so that only a silicon oxide film or a silicon nitride film is formed simply on one surface of a silicon substrate. If the wafer  20  having such an insulating film formed on the silicon substrate is exposed to the plasma  12 , a surface of the insulating film is charged up. Next, when the wafer  20  is taken out from the plasma chamber  11 , charges trapped onto the insulating film remain (as a charge-up residue). This charge-up residue is measured using a noncontact potential measuring instrument to thereby measure a charge-up degree. As can be seen, if the blank wafer is used, the measurement is made after the sensor wafer  20  is taken out from the atmosphere of the plasma  12  and not made when charge-up actually occurs in the atmosphere of the plasma  12 . Therefore, the charge-up may not be measured at real time. Besides, since the insulating film is a plain film without a pattern on the silicon substrate, the charge-up may not be measured in an actual pattern including contact holes. 
       (2) Second Problem 
       [0018]    Since the energy of ions incident on the wafer  20  from the plasma  12  may not be directly measured, the self-alignment bias is monitored and used as an indirect index. Normally, the average value of the self-alignment bias is measured by the voltmeter  15  disposed below the stage  13 . Due to this, an in-plane distribution of the self-alignment bias may not be measured. This second problem will be described in detail. 
         [0019]    As shown in  FIG. 7 , normally, the stage  13  is a conductive electrode. If the self-alignment bias is generated in the atmosphere of the plasma  2 , the self-alignment bias is applied to portions (such as an outer circumference) of the stage  13  to which portions the plasma  12  is exposed. The voltmeter  15  is disposed below and connected to the stage  13 , and measures the self-alignment bias. Due to this, the self-alignment bias is measured while using an entire area of the portions (e.g., the outer circumference) of the stage  13  to which portions the plasma is exposed as an antenna. As a result, how the self-alignment bias differs among plural points on the wafer  20  (on the stage  13 ) and the like may not be measured. In  FIG. 7 , the average value of the self-alignment bias with the outer circumference of the stage  13  set as an antenna (i.e., the average value of each self-alignment biases that possibly slightly differ among various points on the outer circumference of the stage  13 ) is measured. 
       SUMMARY OF THE INVENTION 
       [0020]    According to a first aspect of the present invention, there is provided a plasma monitoring method using a sensor, the sensor comprising:
       a substrate;   a first electrode, the first electrode being a conductive electrode and formed on the substrate and electrically isolated from the substrate;   an insulating film formed on the first electrode;   a contact hole formed in the insulating film and having a depth from a surface of the insulating film to the first electrode; and   a second electrode, the second electrode being a conductive electrode, formed on the surface of the insulating film, and facing a plasma during a plasma process,
 
the plasma monitoring method comprising:
   measuring and monitoring potentials of the first electrode and the second electrode or a potential difference between the first electrode and the second electrode during the plasma process.       
 
         [0027]    According to a second aspect of the invention, there is provided a plasma monitoring system comprising:
       a sensor having a substrate; a first electrode, the first electrode being a conductive electrode and formed on the substrate and electrically isolated from the substrate; an insulating film formed on the first electrode; a contact hole formed in the insulating film and having a depth from a surface of the insulating film to the first electrode; and a second electrode, the second electrode being a conductive electrode, formed on the surface of the insulating film, and facing a plasma during a plasma process; and   a voltmeter measuring potentials of the first electrode and the second electrode or a potential difference between the first electrode and the second electrode during the plasma process.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]      FIG. 1  is a schematic cross-sectional view of a plasma process detection sensor included in a plasma monitoring system according to Example 1 of the present invention; 
           [0031]      FIG. 2  is a schematic configuration diagram of the plasma monitoring system according to Example 1 of the invention; 
           [0032]      FIGS. 3A to 3I  are schematic cross-sectional step views showing each one of the steps of an exemplary method of manufacturing the plasma process detection sensor shown in  FIG. 1 ; 
           [0033]      FIG. 4  is a chart of experimental data showing dependence of a potential of an upper electrode  55  and a potential of a lower electrode  53  shown in  FIG. 1  on a self-alignment bias; 
           [0034]      FIGS. 5A and 5B  are schematic configuration diagrams of a plasma monitoring system  50 A according to Example 2 of the invention; 
           [0035]      FIG. 6  is a schematic cross-sectional view of a plasma process detection sensor according to Example 3 of the invention; 
           [0036]      FIG. 7  is a schematic configuration diagram of a conventional plasma monitoring system; and 
           [0037]      FIG. 8  is a schematic diagram explaining the self-alignment bias. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Example 1 
     Plasma Monitoring System According to Example 1 
       [0038]      FIG. 2  is a schematic configuration diagram of a plasma monitoring system according to Example 1 of the present invention. 
         [0039]    The plasma monitoring system includes a plasma processing apparatus  30 . The plasma processing apparatus  30  is an apparatus that generates plasma  32  within a plasma chamber  31  set in a vacuum by applying an RF bias to the plasma chamber  31 . The plasma processing apparatus  30  performs such processings as etching and film formation on a wafer  40  such as a semiconductor wafer, e.g., a silicon wafer disposed on a conductive stage  33  and serving as a monitoring target workpiece. A voltmeter  35  for self-alignment bias measurement is connected to the stage  33  via a coil  34  for AC voltage component elimination. Two plasma process detection sensors  50  ( 50 - 1 ,  50 - 2 ) are bonded onto a predetermined portion or plural different portions (bonded onto two portions in  FIG. 2 , respectively) on a surface of the wafer  40 . 
         [0040]      FIG. 1  is a schematic cross-sectional view of each of the plasma process detection sensors  50  shown in  FIG. 2  according to Example 1 of the invention. 
         [0041]    This plasma process detection sensor  50  has a substrate (e.g., silicon substrate)  51  and an insulating film  52  having a thickness of about 1.0 μm, made of silicon oxide (SiO2 film), and formed on the silicon substrate  51 . A first electrode (e.g., lower electrode)  53  having a thickness of about 300 nm and made of a conductive matter such as polysilicon (Poly-Si) is selectively formed on the insulating film  52 . An insulating film  54  having a thickness of about 1.0 μm and made of silicon oxide is deposited on the first electrode  53 . A second electrode (e.g., an upper electrode)  55  having a thickness of about 300 nm and made of a conductive material such as polysilicon is selectively formed on the insulating film  54 . A contact hole pattern for plural contact holes  56  to be actually formed in the wafer  44  and each having a circular cross section is formed on the upper electrode  55 . A diameter of the circular cross section of each of the contact holes  56  is about 100 nm. The contact hole  56  has a depth of about 1.3 μm measured from a surface of the upper electrode  55  to a surface of the lower electrode  53 . 
         [0042]    A wiring connection area  57  is opened in an exposed portion of the surface of the insulating film  54  and the surface of the lower electrode  53  is exposed from the wiring connection area  57 . A wiring  58  is connected to the upper electrode  55  and a wiring  59  is connected to the lower electrode  53 , and the two wirings  58  and  59  are led outside of the plasma chamber  31  shown in  FIG. 2  via terminals (not shown), respectively. A voltmeter  60  for measuring potential is connected to the wiring  58  led outside. The voltmeter  60  is connected to a terminal  62  having a reference potential (e.g., ground potential). Similarly, a voltmeter  61  measuring potential is connected to another wiring  59 . The voltmeter  61  is connected to a terminal  63  having a reference potential (e.g., ground potential). 
       Method of Manufacturing the Sensor According to Example 1 
       [0043]      FIGS. 3A to 3I  are schematic cross-sectional views showing each one of the steps of a method of manufacturing each of the sensors  50  shown in  FIG. 1 . The sensor  50  shown in  FIG. 1  is, for example, manufactured by the following steps ( FIGS. 3A to 3I ). 
         [0044]    In an insulating film formation step ( FIG. 3A ), the insulating film  52  having the thickness of about 1.0 μm and made of silicon oxide is formed on the silicon substrate  51  by thermal oxidation. In a conductive film formation step ( FIG. 3B ), a conductive film  53   a  having a predetermined impurity ion concentration, having a thickness of about 300 nm, and made of polysilicon is formed on the insulating film  52  by a chemical vapor deposition (hereinafter, “CVD”) method. In an electrode formation step ( FIG. 3C ), a mask for an electrode pattern which is made of a resist film is formed on the conductive film  53   a  by photolithography. The conductive film  53   a  is then etched by dry etching such as plasma etching to form the lower electrode  53 , and the unnecessary mask is removed by ashing. 
         [0045]    In an insulating film formation step ( FIG. 3D ), the insulating film  54  having the thickness of about 1.0 μm and made of silicon oxide is deposited by the CVD method. In a conductive film formation step ( FIG. 3E ), similarly to the conductive film formation step ( FIG. 3B ), a conductive film  55   a  having a predetermined impurity ion concentration, having the thickness of about 300 nm, and made of polysilicon is formed on the insulating film  54  by the CVD method. In an electrode formation step ( FIG. 3F ), similarly to the electrode formation step ( FIG. 3C ), a mask for an electrode pattern made of a resist film is formed on the conductive film  55   a  by the photolithography, and the conductive film  55   a  is etched by the dry etching such as plasma etching to form the upper electrode  55 . 
         [0046]    In a contact hole formation step ( FIG. 3G ), a resist pattern is formed on the upper electrode  55  by the photolithography, and the upper electrode  55  and the insulating film  54  are etched by a depth up to the surface of the lower electrode  53  using the resist pattern as a mask by the dry etching such as plasma etching, thereby forming the contact hole pattern of plural contact holes  56  each having a circular cross section. The diameter of a circular cross section of each contact hole  56  is about 100 nm and the depth of the contact hole  56  is about 1.3 μm. In a wiring connection area opening step ( FIG. 3H ), the insulating film  54  is etched from the exposed surface to the surface of the lower electrode  53  by the photolithography and by the dry etching to open the wiring connection area  57 . 
         [0047]    Thereafter, in a wiring connection step ( FIG. 3I ), the wirings  58  and  59  are connected to the upper electrode  55  and the lower electrode  53  and to the voltmeters  60  and  61  provided outside of the plasma chamber  31  shown in  FIG. 2 , respectively. 
       Plasma Monitoring Method According to Example 1 
       [0048]    Plural (e.g., two) sensors  50  each having the actual contact hole pattern manufactured as stated above is prepared. The two sensors  50  ( 50 - 1 ,  50 - 2 ) are bonded onto the two different portions on the surface of the wafer  40  shown in  FIG. 2 , respectively, and the resultant sensor-added wafer  40  is mounted on the stage  33  within the plasma chamber  31  in the plasma processing apparatus  30 . An internal pressure of the plasma chamber  31  is set to, for example, 120 mTorr. A mixture gas of CHF3, CF4, N2, and Ar is filled into the plasma chamber  31  as a filler gas. The RF bias at 1600 W is applied to the plasma processing apparatus  30 . Here, the plasma  32  is generated in the plasma chamber  31 , and the wafer  40  is exposed to the plasma  32 . 
         [0049]    If the wafer  40  is exposed to the plasma  32 , charge-up occurs at the bottoms of each of the contact holes  56  in the respective sensors  50  ( 50 - 1  and  50 - 2 ) due to an electron blocking effect (electron shading effect) as shown in  FIG. 1 . Namely, more electrons e are trapped at the upper electrode  55  than at the lower electrode  53 , thereby causing a charge bias between the surface of the contact hole pattern and the bottoms of each of the contact holes  56 . Due to this, the upper electrode  55  and the lower electrode  53  have different potentials. At this time, one or both of the upper electrode  55  and the lower electrode  53  is measured by the voltmeters  60  and  61 , respectively or a potential difference Δ(V2−V1, where V1 is a value indicated by the voltmeter  60  and V2 is a value indicated by the voltmeter  61 ) between the upper electrode  55  and the lower electrode  53  is measured by the voltmeters  60  and  61 , and monitored. 
         [0050]    At this time, if a self-alignment bias Vdc is generated in the atmosphere of the plasma  32 , the self-alignment bias Vdc is applied to the portions (such as the outer circumference) of the conductive stage  33  which portions are exposed to the plasma  32 . Since the voltmeter  35  is disposed below the stage  33  and connected to the stage  33 , the self-alignment bias Vdc is read by the voltmeter  35 . In this way, the self-alignment bias Vdc is measured with the entire area of the plasma-exposed portions of the stage  33  (such as the outer circumference of the stage  33 ) used as an antenna. 
       Advantages of Example 1 
       [0051]      FIG. 4  is a chart of experimental data showing the dependence of the potentials of the upper electrode  55  and the lower electrode  53  shown in  FIG. 1  on the self-alignment bias. In  FIG. 4 , the horizontal axis indicates the self-alignment bias Vdc (V) and the vertical axis indicates the potentials (V) of the upper electrode  55  and the lower electrode  53 . 
         [0052]    According to Example 1, in each of the sensors  50  ( 50 - 1 ,  50 - 2 ), the upper electrode  55  and the lower electrode  53  are provided on the surface of the actual contact hole pattern and the bottoms of the contact holes  56 , and the potentials of the surface of the contact hole pattern and the bottoms of the contact holes  56  are measured simultaneously using the voltmeters  60  and  61 . Due to this, as can be understood from the experimental data shown in  FIG. 4 , the charge-up occurring in the actual contact pattern may be observed as the potential difference between the upper electrode  55  and the lower electrode  53 . Besides, since the potential difference is measured during occurrence of the plasma  32 , the charge-up may be observed at real time. Therefore, process conditions may be optimized and a reduction in yield caused by the charge-up may be prevented. 
         [0053]    Furthermore, as can be understood from the experimental data shown in  FIG. 4 , the potential of the upper electrode  55  has a correlation with the self-alignment bias Vdc measured by the voltmeter  35  shown in  FIG. 2 . Since the sensors  50 - 1  and  50 - 2  are arranged in plural different portions (e.g., two portions) in the plane of the wafer  40 , respectively, the in-plane distribution of the self-alignment bias Vdc may be monitored indirectly. To improve monitoring accuracy, it suffices to increase the number of sensors  50  installed in the plasma processing apparatus  30 . 
       Example 2 
     Configuration of Plasma Monitoring System According to Example 2 
       [0054]      FIGS. 5A and 5B  are schematic configuration diagrams of a plasma monitoring system according to Example 2 of the invention. In  FIGS. 5A and 5B , the same constituent elements as those shown in  FIGS. 1 and 2  according to Example 1 are denoted by the same reference symbols, respectively. 
         [0055]    The sensors  50  (= 50 - 1  to  50 - 5 ) described in Example 1 are bonded onto each of plural (e.g., two) wafers  40 - 1  and  40 - 2 . At this time, the sensor-added wafers  50 - 1  and  50 - 2  are configured so as to differ to each other in the total area of contact holes  56  in the sensors  50  on the wafer  40 - 1  and  40 - 2 , which is defined as (area of cross-sectional circle of one contact hole  56 )×(number of contact holes  56  on the wafer  40 - 1  and  40 - 2 ). For example, in  FIGS. 5A and 5B , the number of sensors  50  arranged on the wafers  40 - 1  and  40 - 2  are different. In  FIG. 5A , the two sensors  50 - 1  and  50 - 2  are arranged in the plane of the wafer  40 - 1 . In  FIG. 5B , the five sensors  50 - 1  to  50 - 5  are arranged in the plane of the wafer  40 - 2 . By such arrangement, the total area of the contact holes  56  in the sensors  50 - 1  to  50 - 5  arranged on the wafer  40 - 2  is 2.5 times as large as that of the contact holes  56  in the sensors  50 - 1  and  50 - 2  arranged on the wafer  40 - 1 . 
       Plasma Monitoring Method According to Example 2 
       [0056]    The two wafers  40 - 1  and  40 - 2 , in which the sensors  50  are arranged, are exposed to the plasma  32  in the same conditions. Namely, the first sensor-added wafer  40 - 1  is disposed within the plasma chamber  31 , exposed to the plasma  32  in certain conditions, and taken out from the plasma chamber  31 . The second sensor-added wafer  40 - 2  is then disposed within the plasma chamber  31  and exposed to the plasma  32  in the same conditions as those for the first wafer  40 - 1 . 
         [0057]    As a result of the exposure of the wafer  40 - 2  to the plasma  32 , charge-up occurs on the bottoms of the contact holes  56  of the sensors  50  by the electron blocking effect (electron shading effect). Due to this, the upper electrode  55  and the lower electrode  53  of each of the sensors  50  have different potentials. At this time, the potential difference between the upper electrode  55  and the lower electrode  53  of each of the sensors  50  arranged on the each of the wafers  40 - 1  and  40 - 2  is measured by the voltmeters  60  and  61 , and monitored. 
         [0058]    Examples of a method of measuring the potential difference between the upper electrode  55  and the lower electrode  53  of a single sensor  50  are as follows. In a first method, the voltmeters  60  and  61  are connected to a single sensor  50  and measure the potentials, respectively. The potentials measured by the voltmeters  60  and  61  are compared with each other (the potential difference is calculated). In a second method, one voltmeter (having two terminals for measuring potentials of two electrodes) is connected to the two electrodes, i.e., the upper electrode  55  and the lower electrode  53  of the single sensor  50 , and the voltage (potential difference) between the two electrodes is directly measured. As can be seen, it is necessary to use two voltmeters per sensor to measure potentials using the voltmeters according to the first method. It is necessary to use a single voltmeter per sensor to measure potentials using the voltmeter according to the second method. Either of the first and second methods may be adopted. 
       Advantages of Example 2 
       [0059]    According to Example 2, by comparing the potential differences measured with respect to the sensor-added wafers  40 - 1  and  40 - 2 , where the respective contact holes  56  of the sensors  50  arranged respectively on the wafers have different total areas, the dependence of the charge-up on the pattern ratio (dependence of the charge-up on the total area of the contact holes  56  per wafer) may be observed. 
         [0060]    Namely, if a plasma etching target area is larger, the amount of plasma gas consumed for the plasma etching is normally larger (because of a large amount of reaction gas reacting with the etching target workpiece). In this case, if the supply amount of the plasma gas relative to the consumption amount is insufficient, plasma etch rate decelerates. The deceleration of the etch rate due to an increase in the consumption amount relative to the supply amount of the plasma gas is referred to as “loading effect”. The loading effect is confirmed by measuring the dependence of the etch rate on the pattern ratio (dependence of the etch rate on the etching target area). 
         [0061]    Similarly to Example 1, according to Example 2, it is considered that the insulating film and the like on inner sidewalls of the contact holes  56  in the sensors  50  are slightly etched. Due to this, if the area of the contact holes  56  present in the wafers  40 - 1  and  40 - 2  is larger (e.g., the number of contact holes  56  is larger or the diameter of each contact hole  56  is larger), the amount of gas reacting with the insulating film and the like on the sidewalls of the contact holes  56  is larger (i.e., the amount of gas consumed in the contact holes  56  out of the plasma  32  within the plasma chamber  31  increases). As a result, the state of the plasma  32  (“plasma state”) within the plasma chamber  31  changes. It is considered, therefore, that charge-up change deriving from the change in the plasma  32  occurs. By observing the dependence of the charge-up on the pattern ratio, therefore, the plasma state may be appropriately monitored. 
       Example 3 
       [0062]      FIG. 6  is a schematic cross-sectional view of a plasma process detection sensor  50 A according to Example 3 of the invention. In  FIG. 6 , the same constituent elements as those shown in  FIG. 1  according to Example 1 are denoted by the same reference symbols, respectively. 
         [0063]    One or more intermediate electrodes may be provided between the upper electrode  55  and the lower electrode  53  in each of the sensors  50  according to Example 1 and Example 2.  FIG. 6  shows an instance of additionally providing one intermediate electrode according to Example 3. 
         [0064]    In the sensor  50 A according to Example 3, an intermediate electrode  64  having a predetermined impurity ion concentration, having a thickness of about 300 nm, and made of polysilicon is formed in the insulating film  54  between the lower electrode  53  and the upper electrode  55 . A voltmeter  66  is connected to the intermediate electrode  64  by a wiring  65 , and connected to a terminal  67  having a reference potential (e.g., ground potential). 
         [0065]    Charge-up occurs onto the inner walls of the contact holes  56  by the plasma  32 . Due to this, if the contact holes  56  are formed in an LSI product or the like by plasma etching, a phenomenon occurs that positive ions h accelerating etching are influenced by the potential of the inner walls of the contact holes  56  so that a path of the positive ions h is curved in a direction of the bottoms of the contact holes  56  and the positive ions h strike against the inner walls of the contact holes  56 , and that the inner walls are etched. If the inner walls of the contact holes  56  are conspicuously etched, problems such as a reduction in product yield occur. Since the potential of the inner walls of the contact holes  56  has an influence on the path of the positive ions h from the plasma  32 , the potential of the inner walls of the contact holes  56  between the upper electrode  55  and the lower electrode  53  may be measured by providing the intermediate electrode  64  and the charge-up in the contact holes  56  may be examined in more detail. 
         [0066]    If two or more intermediate electrodes  64  are provided, the intermediate electrodes  64  may be provided at positions set by dividing equally, e.g., trisecting or quadrisecting the interval between the upper electrode  55  and the lower electrode  53 , respectively or at positions between the upper electrode  55  and the lower electrode  53  at which positions the potential is to be measured, respectively. 
       Modifications 
       [0067]    The invention is not limited to Example 1 to Example 3. Various modifications may be made of the invention and the invention may be carried out in various types of use. Examples of the types of use and modifications include (i) to (iv) as follows. 
         [0068]    (i) In the invention, the configurations, manufacturing methods and the like of the plasma processing apparatus  30  and the sensors  50  and  50 A shown in the drawings may be changed. 
         [0069]    (ii) In  FIG. 2 , the two sensors  50 - 1  and  50 - 2  are provided on the surface of the wafer  40 . Alternatively, one sensor  50 - 1  may be provided on the surface of the wafer  40  or near the wafer  40  (e.g., on the outer circumference of the stage  33 ) depending on usage. Likewise, in  FIGS. 5A and 5B , plural sensors  50  is provided respectively on the surface of each of the wafers  40 - 1  and  40 - 2 . Alternatively, one sensor  50 - 1  may be provided on the wafer  40 - 1  or  40 - 2  while changing the number of contact holes  56  formed in the sensors  50  and the plasma process may be monitored. 
         [0070]    (iii) The plasma monitoring system shown in  FIG. 2  includes the plasma processing apparatus  30 . Alternatively, the plasma monitoring system may be configured to include the sensor  50  and the voltmeters  60  and  61  shown in  FIG. 1  or to include the sensor-added wafer  40  to which one or more sensors  50  is attached and the voltmeters  60  and  61 . At this time, if the voltmeters  60  and  61  are downsized and the downsized voltmeters  60  and  61  and the other circuit components (such as a driving battery and a data storage memory) are included in the sensor  50  or the sensor-added wafer  40 , the plasma monitoring system may be downsized and user-friendliness of the plasma monitoring system is improved. 
         [0071]    (iv) In Example 1 to Example 3, the semiconductor manufacturing process using plasma has been described. However, the invention is applicable to all the other manufacturing processes using plasma than the semiconductor manufacturing process, for example, to a flat panel manufacturing process. 
         [0072]    As can be understood from the foregoing, according to the invention, the second electrode (upper electrode  55 ) and the first electrode (lower electrode  53 ) are provided on the surface of the actual contact hole pattern and the bottoms of the contact holes ( 56 ), respectively, and the potential of the surface of the contact hole pattern and the potential of the bottoms of the contact holes are measured simultaneously. Therefore, the charge-up occurring in the actual contact hole pattern may be observed as the potential difference between the second electrode and the first electrode. Besides, since the potential difference is measured during occurrence of the plasma ( 32 ), the charge-up may be observed at real time. Therefore the process conditions may be optimized and the reduction in yield caused by the charge-up may be suppressed. 
         [0073]    Moreover, the potential of the second electrode has a correlation with the self-alignment bias measured on the plasma processing apparatus. Due to this, if sensors are arranged, for example, in a plurality portions in the plane of the wafer, respectively, the in-plane distribution of the self-alignment bias may be monitored indirectly.