Abstract:
Provided is a technique to automatize a synthesis function of signal charged particles having different energies. A charged particle beam apparatus includes: a charged particle source configured to irradiate a sample with a primary charged particle ray; a first detector configured to detect a first signal electron having first energy from signal charged particles generated from the sample; a second detector configured to detect a second signal electron having second energy from signal charged particles generated from the sample; a first operation part configured to change a synthesis ratio of a signal intensity of the first signal electron and a signal intensity of the second signal electron and to generate a detected image corresponding to each synthesis ratio; a second operation part configured to calculate a ratio of signal intensities corresponding to predetermined two areas of the detected image generated for each synthesis ratio; and a third operation part configured to determine a mixture ratio to be used for acquisition of the detected image on a basis of a change of the ratio of signal intensities.

Description:
TECHNICAL FIELD 
     The present invention relates to a charged particle ray (beam) apparatus that detects signal electrons generated from a sample irradiated with a charged particle beam and generates an image thereof, and a method for measuring a pattern of a surface of the sample based on the image. 
     BACKGROUND ART 
     Higher integration of semiconductor devices have increased the importance to control pattern on the outermost face as well as to perform alignment control (shape control) of an upper-layer pattern and a lower-layer pattern and dimension control of a hole opening. A scanning electron microscope (hereinafter called a SEM) has been used typically for the shape control and the dimension control of such a multilayered device (three-dimensional device). 
     Patent Literature 1 discloses a SEM provided with an energy filter capable of selecting signal electrons detected according to their energies. This type of SEM adjusts a threshold voltage of the energy filter, whereby a contrast image corresponding to a difference in surface potential of the device can be obtained. 
     Patent Literature 2 discloses a SEM including a detector for back scattered electrons and a detector for secondary electrons mounted thereon. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: Japanese Patent No. 4069624 
         Patent Literature 2: JP Patent Publication No. 08-273569 (1996) 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     A method for automatically setting threshold voltage of the energy filter for each observed device has not been known. This makes it difficult to mount an energy filter on a shape control/dimension control apparatus of a semiconductor device that requires automation and speeding up. 
     An image of back scattered electrons has a low contrast. This means the difficulty in measuring a pattern thereof simply based on the image of back scattered electrons. Then, it is reasonable to synthesize the image of back scattered electrons with the image of secondary electrons for shape control and dimension control. 
     Patent Literature 2, however, does not disclose a method of determining an optimum ratio for synthesis of the image of back scattered electrons and the image of secondary electrons. This makes it difficult to mount a synthesis function of the image of back scattered electrons and the image of secondary electrons to a shape control/dimension control device of a semiconductor device that requires automation and speeding up. 
     Solution to Problem 
     To solve the aforementioned problems, the present invention proposes a charged particle beam apparatus including: a charged particle source configured to irradiate a sample with a primary charged particle beam; a first detector configured to detect a first signal electron having first energy from signal charged particles generated from the sample; a second detector configured to detect a second signal electron having second energy from signal charged particles generated from the sample; a first operation part configured to change a synthesis ratio of a signal intensity of the first signal electron and a signal intensity of the second signal electron and to generate a detected image corresponding to each synthesis ratio; a second operation part configured to calculate a ratio of signal intensities corresponding to predetermined two areas of the detected image generated for each synthesis ratio; and a third operation part configured to determine a mixture ratio to be used for acquisition of the detected image on a basis of a change of the ratio of signal intensities. 
     To solve the aforementioned problems, the present invention proposes another charged particle ray (beam) apparatus including: a charged particle source configured to irradiate a sample with a primary charged particle beam; an energy filter configured to separate signal charged particles according to magnitude of energy; a detector configured to detect signal charged particles that have passed through the energy filter; a first operation part configured to change filter voltage to be applied to the energy filter and to generate a detected image corresponding to each filter voltage; a second operation part configured to calculate a ratio of signal intensities corresponding to predetermined two areas of the detected image generated for each filter voltage; and a third operation part configured to determine filter voltage to be used for acquisition of the detected image on a basis of a change of the ratio of signal intensities. 
     Advantageous Effects of Invention 
     According to the present invention, acquisition conditions required for automation of shape control and dimension control of a three-dimensional device can be set automatically. Problems, configurations, and advantageous effects other than those explained above will be made apparent from the following explanations of embodiments. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  shows a first exemplary configuration of a scanning electron microscope. 
         FIG. 2  shows an exemplary configuration of an energy filter. 
         FIG. 3  shows energy distribution of signal electrons. 
         FIG. 4  shows an exemplary structure of a pattern that is a measurement target in Embodiment 1. 
         FIG. 5  is a flowchart to determine setting conditions of a detection system. 
         FIG. 6  shows a condition setting screen (initial image). 
         FIG. 7  shows a condition setting screen (observation screen of reference image). 
         FIG. 8  shows a condition setting screen (area selection screen). 
         FIG. 9  shows a condition setting screen (filter setting screen). 
         FIG. 10-1  shows a relationship between energy distribution of signal electrons and signal intensity. 
         FIG. 10-2  explains how to determine optimum filter voltage when gray level ratio changes in pattern 1. 
         FIG. 10-3  explains how to determine optimum filter voltage when gray level ratio changes in pattern 2. 
         FIG. 10-4  explains how to determine an optimum synthesis ratio when gradation ratio changes in pattern 1. 
         FIG. 10-5  explains how to determine an optimum synthesis ratio when gradation ratio changes in pattern 2. 
         FIG. 11  shows a condition setting screen (observation screen of BSE image). 
         FIG. 12  shows a condition setting screen (edge selection screen). 
         FIG. 13  shows a condition setting screen (edge enhancement screen). 
         FIG. 14  shows an exemplary structure of a pattern that is a measurement target in Embodiment 2. 
         FIG. 15  shows an exemplary image when condition setting is executed. 
         FIG. 16  shows an exemplary structure of a pattern that is a measurement target in Embodiment 3. 
         FIG. 17  shows a second exemplary configuration of a scanning electron microscope. 
         FIG. 18  shows a third exemplary configuration of a scanning electron microscope. 
         FIG. 19  shows a fourth exemplary configuration of a scanning electron microscope. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The following describes embodiments of the present invention, with reference to the drawings. The present invention is not limited to the following embodiments, and can be modified variously within the scope of its technical idea. The following description exemplifies a scanning electron microscope as one of charged particle ray (beam) apparatuses, and the present invention is applicable to a focused ion beam (FIB) microscope as well. 
     Exemplary Configuration 1 
     Overall Configuration 
       FIG. 1  shows a first exemplary configuration of a scanning electron microscope. The following describes an electron beam inspection apparatus that is used for observation of a circuit pattern or a resist pattern formed on a semiconductor wafer, their dimension measurement, shape measurement, inspections and defects reviews. 
     An electron microscope lens barrel  1  includes an electron emitting source  101  attached thereto. The electron emitting source  101  emits a primary electron beam  102  under the control of an electron source controller  201 . The primary electron beam  102  is accelerated by electron source acceleration voltage  202  connected to the electron emitting source  101 . Then, the primary electron beam  102  is optimized for its diameter by one or more focusing lenses  103  and a current limiting aperture  104 . The focusing lenses  103  are controlled by a lens controller  203   a . Then, the primary electron beam  102  is focused on the surface of a sample  106  via an objective lens  105 . The objective lens  105  is controlled by a lens controller  203   b.    
     Negative voltage (retarding voltage)  204  is applied to the sample  106 . The application of the negative voltage (retarding voltage)  204  generates retarding field between the sample  106  and the objective lens  105 . The retarding field decelerates the primary electron beam  102  immediately before the sample, and then the primary electron beam  102  arrives at the surface of the sample  106 . Herein, incident voltage is given by the value of electron source accelerating voltage−retarding voltage. 
     The primary electron beam  102  is deflected by a deflector  107 , thus scanning on the surface of the sample  106 . At this time, signal electrons  108  are generated from the scanning position of the primary electron beam  102 . Herein, the deflector  107  is controlled by a deflection controller  205 . 
     The signal electrons  108  referred to in the present configuration include true secondary electrons (TSE)  108   a  having kinetic energy less than 50 eV and backscattered electrons (BSE)  108   b  having kinetic energy more than 50 eV. 
     The signal electrons  108  generated from the sample  106  are accelerated by the retarding voltage  204  and are incident on an energy filter  109 . The signal electrons  108  that have passed through the energy filter  109  collide with a converting electrode  120 , and so generate second secondary electrons  130 . The second secondary electrons  130  are detected by a detector  110   a . Secondary electrons  131  reflected from the energy filter  109  are detected by a detector  110   b . Positive voltage is applied to the detector  110   a  and the detector  110   b , and electric field generated by such voltage attracts the secondary electrons  130  and  131 . 
     Signals detected at the detector  110   a  and the detector  110   b  are amplified by amplifiers  206   a  and  206   b , respectively, and are input to a synthesis operation part  207 . An image subjected to synthesis operation by the synthesis operation part  207  is displayed on a display  208 . 
     The controllers  201 ,  203   a ,  203   b  and  205  are controlled by a central controller  209  in an integrated way. Control values and adjustment values are stored in a storage device  210 . 
       FIG. 2  shows a basic configuration of the energy filter  109 . The energy filter  109  includes two shield meshes  109   a  and one filter mesh  109   b . These three meshes are provided with openings  111  to let the primary electron beam  102  pass therethrough. 
     To the filter mesh  109   b , a filter power source  211  is connected to apply filter voltage. Similarly to other controllers, the filter power source  211  is controlled by the central controller  209 .  FIG. 2  shows one filter mesh  109   b , but this may be a plurality of filter meshes. Such a plurality of filter meshes  109   b  may be independently connected to the filter power source  211 . 
     Energy Distribution of Signal Electrons 
       FIG. 3  schematically shows energy distribution of signal electrons. The horizontal axis represents energy of the signal electrons and the vertical axis represents occurrence frequency. As described above, the signal electrons include two types of electrons that are different in emission direction and energy. That is, they include true secondary electrons  108   a  and backscattered electrons  108   b.    
     In  FIG. 3 , the energy distribution of the true secondary electrons  108   a  has a peak around from a few eV to 10 eV. On the other hand, the energy distribution of the backscattered electrons  108   b  strongly depends on the average atomic number of the sample  106 . For instance, when the sample  106  has a large average atomic number (Z1), the distribution will be elastically scattered, having a peak at the energy substantially equal to the incident energy of the primary electron beam  102 . When the sample  106  has a small average atomic number (Z2), the distribution has a peak at the energy about half of the incident energy of the primary electron beam  102 . In  FIG. 3 , the waveform of the backscattered electrons  108   b  having smaller energy is represented by the broken line. 
     As shown in  FIG. 3 , the energy distribution of signal electrons actually generated during observation (measurement) varies with materials making up the sample  106 . Therefore the synthesis ratio of two detected signals output from the two detectors  110   a  and  110   b  cannot be decided beforehand. 
     The following embodiments describe the processing functions of the synthesis operation part  207  and the central controller  209  to automatically adjust a synthesis ratio of these two detected signals and automatically implement shape control and dimension control of a three-dimensional device. 
     Embodiment 1 
       FIG. 4  shows an exemplary surface pattern of a three-dimensional device that is assumed as a measurement target. The upper part of the drawing is a plan view of the three-dimensional device, and the lower part is a cross-sectional view of the three-dimensional device. This three-dimensional device has a lowermost layer in which metal  301  is embedded, and a wiring pattern  302  including a plurality of parallel lines is formed at an upper layer. At a gap between the parallel lines of the wiring pattern, an opening  303  is formed at a base layer for continuity with the metal  301  at the lower layer. 
     For measurement of the dimensions of the opening  303  formed in this three-dimensional device, a bottom part of the gap and a metal part of the lower layer have to be made clear in the synthesis image. 
       FIG. 5  shows the procedure that the central controller  209  executes in accordance with a processing program during setting of a detection condition.  FIG. 6  shows an exemplary setting screen displayed on the display  208 . The setting of the detection condition starts with acquisition of an initial image (i.e., reference image) of a measurement pattern. For acquisition of the initial image, the central controller  209  sets the filter voltage at 0 V and sets the synthesis ratio of detected signals at 1:1. 
     For acquisition of the reference image (Step  401 ), the display  208  displays a screen shown in  FIG. 7 . The acquisition of the reference image starts with a click operation by an operator of a “reference image” button  501  on the setting screen. After the acquisition operation ends, an image display part  507  of the display  208  displays a newly acquired reference image (planar image). 
     When the reference image is acquired, then the procedure shifts to area input acceptance processing (Step  402 ). At this step, a screen to accept selection of an area by the operator is displayed so as to allow a pattern edge as a measurement target to be displayed in the image. 
       FIG. 8  shows an exemplary screen corresponding to such processing. In this screen, the operator selects two neighboring regions in the initial image. When the operator presses an “area selection” button  502  in the setting screen, then an area selection button  508  is displayed at a region by the image display part  507 . On the reference image of the image display part  507 , area boxes  509   a  and  509   b  are displayed in a superimposing manner. The following description calls these two area boxes an “area box A” and “area box B.” 
     The area selection button  508  is used for selection of a type of an area box to be set on the reference image. The shape to be used to display each area box may be selected from a template  510 . The operator selects a type of the area box and the shape, and disposes each area box at an appropriate position on the image. Note here that the template may be selected after disposing an area box, and the shape of the area box may be changed. The size of the area box may be changed freely between predetermined minimum and maximum values of the area of the box. 
     When the selection of the area ends, the central controller  209  displays a setting screen of the filter voltage on the display  208  (Step  403 ).  FIG. 9  shows the setting screen of the filter voltage. When the operator presses a “filter setting” button  503  on the setting screen, the central controller  209  starts processing to determine an optimum value of the filter voltage. Herein, if an image necessary for a synthesis ratio of the detected signals described later has been acquired, this setting processing of the filter voltage may be skipped. 
     For automatic setting processing of the filter voltage, a detected signal detected at the detector  110   a  only is used. Herein, the central controller  209  changes the filter voltage from the initial value step by step by a predetermined amount, and every time the filter voltage is changed, the central controller  209  acquires an image of the sample  106 . Letting that the gray levels of the regions surrounded by the area boxes A and B in the image are S A  and S B , respectively, then every time the filter voltage is changed and a corresponding image is acquired, the central controller  209  calculates the gray level ratio S A /S B . The central controller  209  uses this gray level ratio as an evaluation value, and optimizes the filter voltage as in the following procedure. 
     Referring now to  FIGS. 10-1  to  10 - 3 , a method to set an optimum value of the filter value is described below.  FIG. 10-1  shows a relationship between energy of signal electrons and signal intensity. The horizontal axis shows the energy of signal electrons and the vertical axis represents the signal intensity. Herein, the metal part (place selected by the area box A) at the lower layer of the three-dimensional device is often made of heavy metal, and its surrounding part (place selected by the area box B) is often made of light metal. 
     In such a case, at a part of the detected signals for secondary electrons having low energy, a difference in intensity between them is small (region [I] in the drawing). On the other hand, at a part of the detected signals for backscattered electrons having energy close to the incident energy E0, the detection is more at the part of the area box A than at the part of the area box B (region [II] in the drawing). 
     As described above, the central controller  209  activates the energy filter  109  and sets finite filter voltage (in the drawing, ΔVf). Then, a signal having energy lower than ΔVf is reflected from the energy filter  109 , and a signal having energy higher than ΔVf only is detected. 
     This means that ΔVf that is sufficiently large enables no detected signals to be detected from region [I] in  FIG. 10-1 . At this time, the gray level ratio S A /S B  increases. On the other hand, in the case of a too large ΔVf, absolute signal intensity of both of signals from the area box A and from the area box B becomes small, thus failing in the distinction from noise during signal detection. 
     Letting that the gradation (gray level) on the image corresponding to the amplitude of noise is N and the value five times it is set as a noise threshold, ΔVf can be determined so that the gray level ratio S A /S B  is maximized while keeping the gray level of the area box A&gt;5N and the gray level of the area box B&gt;5N. Although the noise threshold is specified as five times or more the noise amplitude N, this constant may be variable. 
     Referring to  FIGS. 10-2  and  10 - 3 , the following describes a method to optimize ΔVf.  FIG. 10-2  shows an exemplary case where larger ΔVf makes the gray level ratio S A /S B  also larger. In this case, too large ΔVf makes S B  smaller than the noise threshold. Therefore when the gray level ratio S A /S B  changes in this way, the central controller  209  sets ΔVf making S B  equal to the noise threshold as the optimum value of the filter voltage. 
       FIG. 10-3  shows an exemplary case where the gray level ratio S A /S B  has a local maximum value with respect to ΔVf. In the range of ΔVf where both of S A  and S B  do not fall below the noise threshold, S A /S B  becomes maximum. Then the central controller  209  sets ΔVf where S A /S B  becomes maximum as the optimum value of the filter voltage. 
     The above procedure is only to quantify the evaluation value (gray level ratio S A /S B ) and determine the magnitude relationship, and so it is easy to automatize the procedure. The central controller  209  sets the thus found filter voltage as the optimum value. 
     When the setting of the filter voltage ends, the central controller  209  displays an acquired image of a detected image of backscattered electrons (BSE image) (Step  404 ).  FIG. 11  shows a setting screen of the BSE image. When the operator presses a “BSE image” button  504 , the central controller  209  executes the acquisition processing of the BSE image. 
     In this case, the central controller  209  uses a detected signal detected at the detector  110   a  only. The central controller  209  acquires an image using the filter voltage optimized at Step  403 . When acquiring the new image based on the optimized filter voltage, the central controller  209  displays the acquired image on the image display part  507 . 
     When the BSE image is acquired, the central controller  209  executes processing to allow the operator to set and input a region of an edge to be observed in the BSE image (Step  405 ).  FIG. 12  shows an edge selection screen. When the operator presses an “edge selection” button  505  on the setting screen, an edge selection button  511  is displayed at a region by the image display part  507 . Then on the BSE image of the image display part  507 , an area box  509   c  is displayed. This area box to designate the edge is called an “area box C” in the following description. The operator selects the shape of the area box from the template  510 . Then the operator disposes the area box C at an appropriate position on the BSE image. The size of the area box C preferably is changed freely. 
     When the setting of the area box C ends, the central controller  209  displays a setting screen to automatically set the mixture ratio of two detected signals on the display  208  (Step  406 ).  FIG. 13  shows a setting screen for mixture ratio. When the operator presses an “edge enhancement” button  506  on the setting screen, the central controller  209  starts processing to automatically determine the mixture ratio of the two detectors  110   a  and  110   b.    
     When starting the processing, the central controller  209  changes the mixture ratio of the detected signal of the detector  110   a  and the detected signal of the detector  110   b  step by step, and every time the mixture ratio is changed, the central controller  209  acquires a synthesized image of the two detected signals based on the set mixture ratio. 
     Let that the gradation (gray level) of the area box C in the image is S C . Then the central controller  209  calculates the gray level ratio S C /S A . The central controller  209  uses this gray level ratio as an evaluation value, and determines an optimum mixture ratio as in the following procedure. Alternatively, the region to be compared with the area box C about the gray level may be the area box B, and the gray level ratio S C /S B  may be calculated as the evaluation value. 
     Referring to  FIGS. 10-4  and  10 - 5 , the following describes a method to optimize the mixture ratio.  FIG. 10-4  shows an exemplary case where a larger mixture ratio makes the gray level ratio S C /S A  also larger. A too large mixture ratio makes S A  smaller than the noise threshold. Therefore when the gray level ratio S C /S A  changes in this way, the central controller  209  sets the mixture ratio making S A  equal to the noise threshold as the optimum value. 
     On the other hand,  FIG. 10-5  shows an exemplary case where the gray level ratio S C /S A  has a local maximum value with respect to the mixture ratio. In the range of the mixture ratio where both of S C  and S A  do not fall below the noise threshold, S C /S A  becomes maximum. Then, the central controller  209  sets the mixture ratio where S C /S A  becomes maximum as the optimum value. 
     Summary 
     With the above procedure, the central controller  209  automatically optimizes the mixture ratio of the filter voltage and the detected signals, and acquires an image under the condition where the gray level ratio becomes the best. This enables automatic setting of the optimum mixture ratio of true secondary electrons and backscattered electrons for the sample  106  as the measurement target. This enables observation of the opening  303  that is the second darkest in the image and has a small difference in gradation (gray levels) from the darkest region. Then, even for a three-dimensional device having an unknown structure, the dimensions of the opening  303  can be measured under the optimum condition. 
     In the above embodiment, the filter voltage is set at 0 V and the synthesis ratio of detected signals is set at 1:1 during the acquisition of the reference image, and these values may be freely set by an operator. 
     In the above embodiment, when determining the filter voltage at Step  403 , an image during the processing is not displayed. The list of each image when the filter voltage is changed step by step may be displayed. Then the operator who checks the list of these images may set the condition of any selected image as the filter setting voltage. 
     In the above embodiment, when determining the synthesis ratio of two detected signals at Step  406 , an image during the processing is not displayed. Alternatively, the list of each image when the synthesis ratio is changed step by step may be displayed. Then the operator who checks the list of these images may set the condition of any selected image as the synthesis ratio. 
     Embodiment 2 
       FIGS. 14(   a ) to ( c ) show another exemplary surface pattern of a three-dimensional device that is assumed as a measurement target. This assumed three-dimensional device is a lattice-shaped device where a horizontally extending line and space pattern  601  (hereinafter called “lower layer line  601 ”) and a vertically extending line and space pattern  602  (hereinafter called “upper layer line  602 ”) are laminated. 
     The area of a gap part  603  other than a region where the upper layer line  602  and the lower layer line  601  cross each other determines the device characteristics, and so the dimension control is required thereto. Therefore the horizontal width of the gap part  603  has to be measured, and the edge of the lower layer line  601  and the edge of the upper layer line  602  have to be clarified while emphasizing the contrast between the lower layer line  601  and the gap part  603 . 
     When the height of the lines of the upper layer line  601  is larger than the width between lines, the space between lines is like a deep groove, thus making it difficult to determine the presence or not of the lower layer line  601 . The method shown in  FIG. 5  enables the optimization of the setting parameters in this device also.  FIGS. 15(   a ) to ( c ) show an exemplary image when Step  401  to Step  406  are executed. As is understood from the comparison between  FIGS. 15(   a ) and ( c ),  FIG. 15(   c ) subjected to adjustment of the mixture ratio includes a horizontal stripe pattern  1501  that is not displayed in  FIG. 15(   a ). 
     Embodiment 3 
     The three-dimensional device that is measurable as a measurement target is not limited to the three-dimensional devices shown in  FIG. 4  and  FIG. 14 . The measurement is suitable to a three-dimensional device having a pattern structure shown in  FIGS. 16(   a ) to ( c ) as well. The assumed three-dimensional device shown in these drawings has a shape as shown in  FIG. 15(   b ) such that a base layer  710  is separated in the direction where a pattern  711  is aligned. As shown in  FIG. 15(   b ), this pattern structure includes three layers having different height positions viewed from the irradiation direction of the primary electron beam. This means that the structure including three typical levels of brightness is assumed for detected signals. 
     Herein, the step height between the base layer  710  and its substrate is small. Therefore, a difference in brightness is small. Then when there is a need at Step  401  for the acquisition of a reference image to emphasize the shape of a darkest part  701  and emphasize an outline  702  of the border between the darkest part and the adjacent part, the aforementioned method of the present invention is applicable. 
     Actually Step  403  corresponds to a step to emphasize the shape of the darkest part  701  and Step  406  corresponds to a step to emphasize the outline  702 . 
     Exemplary Configuration 2 
       FIG. 17  shows an exemplary structure of a scanning electron microscope according to another exemplary configuration.  FIG. 17  shows elements corresponding to those of  FIG. 1  with the same reference numerals. In this exemplary configuration, an annular detector  801   a  having a large opening with respect to the central axis detects signal electrons  108  extending over a wide range from the central axis like backscattered electrons (BSE)  108   b . On the other hand, a disk-shaped detector  801   b  having an opening at the center to let a primary electron beam  102  pass therethrough detects signal electrons  108  accelerated by retarding voltage  204  and not extending over a wide range from the central axis like true secondary electrons (TSE)  108   a.    
     Signals detected at the detector  801   a  and the detector  801   b  are amplified by amplifiers  206   a  and  206   b , respectively, and then these signals undergo image operation at the synthesis operation part  207  and are displayed on the display  208 . 
     The detectors  801   a  and  801   b  are disposed at different heights in the central axis direction. Below the detected face of the detector  801   a , an annular filter mesh  802  having a shape similar to that of the detector is disposed. To the filter mesh  802 , a filter power source  211  is connected, and filter voltage is controlled by a central controller  209 . 
     In this way, when the scanning electron microscope includes a detector to detect signal electrons that have passed through the energy filter and a detector to detect signal electrons that do not pass through nor collide with the energy filter, the technique described in the exemplary configuration 1 is directly applicable to such a scanning electron microscope as well. 
     Exemplary Configuration 3 
       FIG. 18  shows still another exemplary configuration of a scanning electron microscope. Similarly to the exemplary configuration 1, the scanning electron microscope according to this exemplary configuration includes an energy filter  109  that separates signal electrons  108  generated from the sample  106  according to their energy levels. 
     In the present exemplary configuration, however, the scanning electron microscope includes a detector  901  only that detects secondary electrons that have passed through the energy filter  109 . In this case, signals detected at the detector  901  are amplified by an amplifier  206  and are displayed on a display  208 . 
     In the present exemplary configuration, the energy filter  109  is configured as in  FIG. 2 . When the detector  901  is configured like a disk-shape having an opening letting a primary electron beam  102  pass therethrough, the optimum value of the filter power source  211  to acquire a backscattered electrons (BSE) image due to a difference in material contrast can be determined easily by executing Steps  401  to  403  shown in  FIG. 5 . 
     Exemplary Configuration 4 
     When two detectors can detect separately two types of signal electrons that are different in emission angle and energy level, the scanning electron microscope may not have an energy filter as in  FIG. 19  showing elements corresponding to those in  FIG. 1  with the same reference numerals. In this case, there is no need to adjust an energy filter. In this case also, the mixture ratio of two detected signals can be automatically determined by executing Steps  404  to  406  shown in  FIG. 5 . 
     Exemplary Configuration 5 
     All of the aforementioned exemplary configurations describe the configuration of applying the retarding voltage  204 . However, the retarding voltage  204  may not be applied. Alternatively, in another configuration, the retarding voltage  204  is not applied, an electrode to apply positive voltage is disposed immediately above the sample, and signal electrons  108  output from the sample  106  is drawn upward at an accelerated rate. Needless to say, in still another configuration, the retarding voltage  204  may be applied, the signal electrons  108  may be drawn upward, and an electrode to apply positive voltage for acceleration may be disposed. 
     Other Exemplary Configurations 
     The present invention is not limited to the above-described embodiments, and may include various modification examples. For instance, the entire detailed configuration of the embodiments described above for explanatory convenience is not always necessary for the present invention. A part of one embodiment may be replaced with the configuration of another embodiment, or the configuration of one embodiment may be added to the configuration of another embodiment. The configuration of each embodiment may additionally include another configuration, or a part of the configuration may be deleted or replaced. 
     The above-described configurations, functions, processing parts, processing means and the like, a part or the entire of them, may be implemented by an integrated circuit or other types of hardware, for example. Alternatively, the above-described configurations, functions and the like may be implemented by software using a processor that interprets a program to implement these functions and executes the program. Information such as programs, tables and files to implement these functions may be placed on a recording device such as a memory, a hard disk or a SSD (Solid State Drive), or a recording medium such as an IC card, a SD card or a DVD. 
     Control lines and information lines illustrated are those considered necessary for the description, and all of the control lines and information lines necessary for the product are not always shown. It can be considered that almost all configurations are mutually connected actually. 
     REFERENCE SIGNS LIST 
     
         
           1  Electron microscope lens barrel 
           101  Electron emitting source 
           102  Primary electron beam 
           103  Focusing lens 
           104  Current limiting aperture 
           105  Objective lens 
           106  Sample 
           107  Deflector 
           108  Signal electrons 
           108   a  True secondary electrons 
           108   b  Backscattered electrons 
           109  Energy filter 
           110   a  Detector 
           110   b  Detector 
           120  Converting electrode 
           130 ,  131  Secondary electrons 
           201  Controller 
           203   a ,  203   b  Controller 
           205  Controller 
           204  Retarding voltage 
           206   a ,  206   b  Amplifier 
           207  Synthesis operation part 
           208  Display