fig2 is a schematic configuration diagram of a plasma monitoring system according to example 1 of the present invention . 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 fig2 , respectively ) on a surface of the wafer 40 . fig1 is a schematic cross - sectional view of each of the plasma process detection sensors 50 shown in fig2 according to example 1 of the invention . 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 . 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 fig2 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 ). fig3 a 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 fig1 . the sensor 50 shown in fig1 is , for example , manufactured by the following steps ( fig3 a to 3i ). in an insulating film formation step ( fig3 a ), 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 ( fig3 b ), 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 ( fig3 c ), 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 . in an insulating film formation step ( fig3 d ), 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 ( fig3 e ), similarly to the conductive film formation step ( fig3 b ), 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 ( fig3 f ), similarly to the electrode formation step ( fig3 c ), 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 . in a contact hole formation step ( fig3 g ), 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 ( fig3 h ), 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 . thereafter , in a wiring connection step ( fig3 i ), 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 fig2 , respectively . 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 fig2 , 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 . 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 fig1 . 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 . 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 . fig4 is a chart of experimental data showing the dependence of the potentials of the upper electrode 55 and the lower electrode 53 shown in fig1 on the self - alignment bias . in fig4 , 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 . 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 fig4 , 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 . furthermore , as can be understood from the experimental data shown in fig4 , the potential of the upper electrode 55 has a correlation with the self - alignment bias vdc measured by the voltmeter 35 shown in fig2 . 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 . fig5 a and 5b are schematic configuration diagrams of a plasma monitoring system according to example 2 of the invention . in fig5 a and 5b , the same constituent elements as those shown in fig1 and 2 according to example 1 are denoted by the same reference symbols , respectively . 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 fig5 a and 5b , the number of sensors 50 arranged on the wafers 40 - 1 and 40 - 2 are different . in fig5 a , the two sensors 50 - 1 and 50 - 2 are arranged in the plane of the wafer 40 - 1 . in fig5 b , 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 . 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 . 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 . 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 . 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 . 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 ). 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 . fig6 is a schematic cross - sectional view of a plasma process detection sensor 50 a according to example 3 of the invention . in fig6 , the same constituent elements as those shown in fig1 according to example 1 are denoted by the same reference symbols , respectively . 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 . fig6 shows an instance of additionally providing one intermediate electrode according to example 3 . 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 ). 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 . 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 . 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 . ( 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 . ( ii ) in fig2 , 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 fig5 a 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 . ( iii ) the plasma monitoring system shown in fig2 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 fig1 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 . ( 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 . 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 . 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 .