Patent Publication Number: US-2022230845-A1

Title: Charged Particle Beam Device

Description:
TECHNICAL FIELD 
     The present disclosure relates to a charged particle beam device. 
     BACKGROUND ART 
     A scanning electron microscope is one of charged particle beam devices. In the scanning electron microscope, an electron beam emitted from an electron source toward a sample is deflected by a scanning coil for two-dimensionally scanning the sample, and is further focused on the sample by an objective lens. At a radiation position, secondary electrons and backscattered electrons are generated as signal electrons from the sample. The generated signal electrons are detected by a detector, and collected information on the signal electrons is mapped in synchronization with a scanning position to obtain an observation image of the sample. 
     Generally, the secondary electrons and the backscattered electrons are substantially distinguished with reference to energy emitted from the sample. Secondary charged particles having energy of 50 eV or less are referred to as the secondary electrons, while secondary charged particles having energy of 50 eV or more are referred to as the backscattered electrons. It is known that each type of the signal electrons have different information on the sample since these signal electrons have different generation principles. 
     In a three-dimensional measurement of a deep hole structure or a deep trench structure on a surface of the sample, information on the sample is obtained from signal electrons generated at a bottom surface or an edge portion. Among the signal electrons, information on the bottom surface and depth is obtained from the backscattered electrons. This is because the backscattered electrons have an angular characteristic of being emitted in a specular reflection direction with respect to an angle at which a primary electron beam is incident on the sample. Information on the edge portion is obtained from the secondary electrons. 
     In order to obtain information on a depth direction, it is necessary to selectively acquire signals of the backscattered electrons, and measurement accuracy deteriorates when the secondary electrons are mixed. Therefore, an improvement in accuracy of the three-dimensional measurement is linked to discriminating the backscattered electrons from among detected signal electrons with high accuracy by an energy difference between the backscattered electrons and the secondary electrons. 
     Patent Literature 1 discloses a method for efficiently detecting secondary electrons and backscattered electrons from a sample using a plurality of detectors. In this method, signal electrons from the sample are directly detected by a first detector provided with a thin metal film on a detection surface. The first detector includes a scintillator detector. Further, tertiary electrons generated on the detection surface of the first detector are detected by a second detector. By taking a sum of detection signals of all the detectors, highly efficient detection is implemented. 
     Patent Literature 1 further discloses a method for distinguishing the secondary electrons from the backscattered electrons, in which the first detector is taken as a backscattered electron detector while the second detector is taken as a secondary electron detector. In this literature, an energy region where the tertiary electrons are generated with high efficiency on the detection surface of the first detector has about 1 kV of energy, and an energy region where a scintillator emits light with high efficiency has several kV of energy. 
     Patent Literature 2 discloses a method for acquiring an image by discriminating energy of signal electrons using a plurality of detectors. In this method, the plurality of detectors are disposed at the same solid angle, each detector detects the same number of signal electrons, and energy discrimination is performed using a difference in energy sensitivity between the detectors. The difference in energy sensitivity can be caused by changing a thickness of a thin film provided on a detection surface of a detector, and an image after energy discrimination is obtained by taking a difference between detection signals of the detectors. 
     Patent Literature 3 discloses a method in which backscattered electrons passed through an aperture for passing a primary electron beam are deflected toward a detector by a deflector, and secondary electrons are removed by an energy filter installed in front of the detector to selectively detect the backscattered electrons. When the primary electron beam is perpendicularly incident on a sample, an angle between a direction of signal electrons generated from the sample and a sample surface is defined as an elevation angle. That is, the elevation angle of the signal electron specularly reflected with respect to the primary electron beam is 90°. In the method of Patent Literature 3, since the backscattered electrons passed through the aperture for passing the primary electron beam are detected, the backscattered electrons having an elevation angle of around 90° are detected. 
     Patent Literature 4 describes a configuration that can detect backscattered electrons in a wide range of elevation angles by a detector used for the backscattered electrons disposed between an objective lens and a sample. 
     CITATION LIST 
     Patent Literature 
     PTL 1: JP-A-10-294074 
     PTL 2: WO2011/089955 
     PTL 3: WO2020/095346 
     PTL 4: Japanese Patent No. 5965819 
     SUMMARY OF INVENTION 
     Technical Problem 
     In the discrimination method disclosed in Patent Literature 1, a threshold value of energy of electrons that can be transmitted is determined by a thickness of a metal film. Accordingly, distinguishing is implemented only under a condition that energy of the secondary electrons is lower than the threshold value while energy of the backscattered electrons is higher than the threshold value. Therefore, in order to perform distinguishing, conditions such as energy of the primary electron beam and a voltage of an electrode used for electro-optical control cannot be set at will. 
     In the method disclosed in Patent Literature 2, the energy discrimination is performed by detecting only signal electrons having energy for transmitting the metal film as in Patent Literature 1 and performing difference calculation on signals from all the detectors. This method is on a premise that number distribution of the signal electrons is axisymmetric with respect to the primary electron beam, and the same number of signal electrons are always required to reach the plurality of detectors. However, actual signal electrons may show non-axisymmetric distribution with respect to a trajectory of the primary electron beam. In such a case, it is difficult to implement energy discrimination with high accuracy using a technique of this literature. 
     In the three-dimensional measurement, the backscattered electrons obtained by perpendicularly reflecting the primary electron beam at a bottom surface are distributed with an elevation angle of around 90°, and thus detection of the signal electrons around 90° is important. On the other hand, a range of elevation angles required for the measurement varies according to an aspect ratio of the structure, and detection in a wider range of elevation angles is required for a structure having a lower aspect ratio. The configuration in Patent Literature 3 can only detect the backscattered electrons at the elevation angle of around 90°, and is difficult to measure a structure having a low aspect ratio. 
     Patent Literature 3 further describes a configuration in which the energy filter is placed in front of the detector to remove the secondary electrons and detect only the backscattered electrons. In order to obtain a secondary electron image and a backscattered electron image by this detection method, images are needed to be acquired in a state in which the energy filter is on and a state in which the energy filter is off, and it takes time to acquire the images. 
     In the configuration of Patent Literature 4, although backscattered electrons in a wide range of elevation angles can be detected, backscattered electrons in an elevation angle of around 90° cannot be detected. 
     The disclosure is made in view of the above problems in the related art, and an objective of the disclosure is to provide a charged particle beam device capable of detecting signal charged particles in a wide range of elevation angles from a large elevation angle to a small elevation angle and distinguishing detection signals of the signal charged particles between backscattered charged particles and secondary charged particles regardless of distribution of the signal charged particles. 
     Solution to Problem 
     The charged particle beam device according to the disclosure includes a first detector that detects the secondary charged particles or the backscattered charged particles and a second detector that detects tertiary charged particles generated from the first detector, and generates an observation image of a sample using a signal value obtained by subtracting at least a part of a second detection signal output by the second detector from a first detection signal output by the first detector, or subtracting at least a part of the first detection signal from the second detection signal. 
     Advantageous Effect 
     According to the charged particle beam device of the disclosure, the signal charged particles in a wide range of elevation angles can be detected, and the detection signals can be distinguished between the backscattered charged particles and the secondary charged particles regardless of the distribution of the signal charged particles. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram of trajectories of signal electrons  12  when a primary electron beam  3  is radiated onto a bottom surface of a sample recess  200 . 
         FIG. 2  is a schematic diagram showing a state in which the signal electrons collide with inner walls of a sample. 
         FIG. 3  is a configuration diagram of a scanning electron microscope  101  according to a first embodiment. 
         FIG. 4  is a schematic plan view showing a state in which a first detector  13  and a second detector  16  detect the signal electrons  12 . 
         FIG. 5  is a plan view showing a configuration example in which a plurality of second detectors  16  are disposed along an outer periphery of the first detector  13 . 
         FIG. 6  is a configuration diagram of the scanning electron microscope  101  according to a second embodiment. 
         FIG. 7  is a schematic diagram showing a configuration of a detection system provided in the scanning electron microscope  101  according to a third embodiment. 
         FIG. 8  is a plan view showing an arrangement of the second detectors  16  in the third embodiment. 
         FIG. 9  shows an example of a screen of a user interface presented by the scanning electron microscope  101 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Problems in Related Art 
     Hereinafter, a problem in a charged particle beam device in the related art will be described first, and then a charged particle beam device according to the disclosure will be described. 
       FIG. 1  is a schematic diagram of trajectories of signal electrons  12  when a primary electron beam  3  is radiated onto a bottom surface of a sample recess  200 . In a case of a structure with a high aspect ratio, elevation angles of the signal electrons  12  emitted to an outside of a sample is limited to around 90°. On the other hand, in a case of a structure with a low aspect ratio, the signal electrons  12  are emitted in a wider range of elevation angles. Therefore, in order to accurately measure the structure with a low aspect ratio, it is necessary to detect signal electrons in a wide range of elevation angles. 
       FIG. 2  is a schematic diagram showing a state in which the signal electrons collide with inner walls of the sample. As shown in  FIG. 2 , when the primary electron beam  3  is radiated onto the bottom surface of the sample recess  200 , a part of the signal electrons  12  collide with the inner walls of the sample and are not emitted to the outside of the sample. As a result, the signal electrons  12  show non-axisymmetric distribution with respect to a trajectory of the primary electron beam. When the signal electrons  12  are distributed in this manner, a difference in the number of detected signal electrons occurs among a plurality of detectors, and energy discrimination cannot be performed by the difference between the detectors. 
     First Embodiment 
       FIG. 3  is a configuration diagram of a scanning electron microscope  101  according to the first embodiment of the present disclosure. A cathode voltage  51  is applied to an electric field emission cathode  1 , and an extracting voltage  52  is applied to an extraction electrode  2 . As a result, an extraction electric field is formed, and a primary electron beam  3  is generated. 
     The primary electron beam  3  is focused by a condenser lens  4 , and is deflected by an upper scanning deflector  5  and a lower scanning deflector  6  to two-dimensionally scan a sample  10 . In order to control an intensity and an aperture angle of the primary electron beam  3 , an objective aperture  7  is placed between the extraction electrode  2  and the condenser lens  4 . 
     The primary electron beam  3  after deflection is further accelerated by a post-stage acceleration cylinder  11  to which a post-stage acceleration voltage  54  provided in a circuit of an objective lens  8  is applied, and is focused on the sample by the objective lens  8 . 
     The signal electrons  12  such as secondary electrons and backscattered electrons are generated from a radiation position of the primary electron beam  3  on the sample  10 . The signal electrons  12  are accelerated by an electric field between a negative retarding voltage  53  applied to a sample stage  17  and the post-stage acceleration cylinder  11 . 
     The signal electrons  12  collide with a reflection plate  14  formed on a surface of a first detector  13 . A part of the electrons collided with the reflection plate  14  are converted into tertiary electrons  15 , and electrons transmitting the reflection plate  14  are converted into first light  18  by a conversion element (scintillator) in the first detector  13 . The first light is taken into a photomultiplier tube and detected as an electric signal. 
     The tertiary electrons  15  are guided to a second detector  16  and are similarly converted into second light  19  by a conversion element (scintillator). The second light  19  is then detected as an electric signal by a photomultiplier tube. At this time, by applying a voltage  55  to a detection surface of the first detector  13 , the generated tertiary electrons  15  can be accelerated and detection efficiency of the second detector  16  can be improved. By applying a voltage  56  to a detection surface of the second detector  16 , the tertiary electrons  15  can be accelerated, and detection efficiency of the second detector  16  can be improved. 
     A deflection electrode  21  applies an electric field orthogonal to an axis of the primary electron beam  3 . A deflection coil  22  applies a magnetic field orthogonal to the axis of the primary electron beam  3  and the electric field. As a result, the detection efficiency can be improved by deflecting the tertiary electrons  15  in a direction of the second detector  16 . In addition, deflection of the primary electron beam  3  by the magnetic field can be cancelled, and straightness of the primary electron beam  3  can be maintained. 
     Regarding to a size of the detection surface of the first detector  13 , it is desirable to widen the detection surface in order to detect the signal electrons  12  in a wide range of elevation angles. 
     Hereinafter, a procedure for obtaining an observation image of the sample will be described. By inputting necessary information from an input and output unit  104 , an operation program is generated and stored in a storage unit  105 . In accordance with the stored operation program, an overall control unit  102  and a signal processing unit  103  are controlled. 
     The overall control unit  102  operates the scanning electron microscope  101  and the signal processing unit  103  in accordance with control conditions according to the operation program. 
     A first output signal output from the first detector  13  and a second output signal output from the second detector  16  are taken into the signal processing unit  103 . The signal processing unit  103  obtains a backscattered electron signal by calculating the first output signal and the second output signal. An equation for subtracting the second output signal from the first output signal is represented as a synthesis equation. This synthesis equation is determined based on first conversion efficiency (sensitivity α 1 ) indicating efficiency with which the conversion element of the first detector  13  converts the secondary electrons into a part of the first light  18 , second conversion efficiency (sensitivity β 1 ) with which the backscattered electrons is similarly converted into the first light  18 , and third conversion efficiency (sensitivity α 2 ) and fourth conversion efficiency (sensitivity β 2 ) with which the secondary electrons and the backscattered electrons are similarly converted into the second light  19  by the conversion element of the second detector  16 . 
     When a detection signal of the first detector  13  is X 1  and a detection signal of the second detector  16  is X 2 , the synthesis equation is shown as an Equation 1 as follows. The sensitivities α 1 , β 1 , α 2 , and β 2  are determined by voltage values of the cathode voltage  51 , the extracting voltage  52 , the retarding voltage  53 , the voltage  55  applied to the detection surface of the first detector, and the voltage  56  applied to the detection surface of the second detector, and material characteristics of the reflection plate  14 . Therefore, synthesis ratios under these voltage conditions are stored in the storage unit  105  in advance, and are determined based on the operation program. When an image of the secondary electrons is obtained, an Equation 2 as follows may be used. A value obtained by the synthesis equation can be output from the overall control unit  102  to the input and output unit  104 . 
       BACKSCATTERED ELECTRON SIGNAL=−α2/(α1β2−α2β1) X 1+α1/(α1β2−α2β1) X 2   Equation 1
 
       SECONDARY ELECTRON SIGNAL=β2/(α1β2−α2β1) X 1−β1/(α1β2−α2β1) X 2   Equation 2
 
       FIG. 4  is a schematic plan view showing a state in which the first detector  13  and the second detector  16  detect the signal electrons  12 . The detection surface of the first detector  13  is the reflection plate  14 . Cross marks indicate collision positions of the signal electrons  12 . In measurement of a recess having a three-dimensional shape, as shown in  FIG. 4 , it is considered that the signal electrons  12  are non-asymmetrically distributed with respect to the primary electron beam  3 . 
     In a configuration of the detector shown in  FIG. 4 , it can be considered that one signal electron  12  is detected by the first detector  13  and the second detector  16 . Therefore, when the number of signal electrons detected by the first detector and the number of signal electrons detected by the second detector are added up, it can be considered that the same number of signal electrons are incident on the detectors regardless of distribution of the signal electrons. As a result, backscattered electron signals (or secondary electron signals) can be obtained even when the signal electrons  12  are non-asymmetrically distributed. 
     When an image of backscattered electrons emitted at an elevation angle of around 90° is required in measurement of a structure with a high aspect ratio, signal electrons passed through an aperture hole of the first detector  13  are deflected by a Wien filter deflector  20 , and an upper detector  23  detects the resultant backscattered electrons. By adding up the backscattered electrons output from the upper detector  23  according to Equation 1, an image of the backscattered electrons emitted at an elevation angle of around 90° can be obtained. 
       FIG. 5  is a plan view showing a configuration example in which a plurality of second detectors  16  are disposed along an outer periphery of the first detector  13 . When the detection surface of the first detector  13  is large, the tertiary electrons  15  that cannot be detected by only the second detector  16  disposed at one place are emitted. In this case, as shown in  FIG. 5 , the plurality of second detectors  16  may be disposed around the first detector  13 , and detection signals of the second detectors  16  may be added up in the signal processing unit  103 . 
     First Embodiment: Overview 
     The scanning electron microscope  101  according to the first embodiment includes the first detector  13  with the detection surface serving as the reflection plate, and the second detector  16  that detects tertiary charged particles generated from the reflection plate. As a result, signal charged particles that hit the reflection plate are detected by two detectors. Therefore, the number of the signal charged particles detected by each detector can be equaled. That is, the plurality of detectors detect the same number of signal charged particles regardless of distribution of the signal charged particles. Thus, energy discrimination can be implemented using a difference in detection signal intensity based on a difference in energy sensitivity between the detectors. 
     In the scanning electron microscope  101  according to the first embodiment, signal charged particles in a correspondingly wide range of elevation angles can be detected by enlarging the detection surface of the first detector  13  in a possible range. However, it is necessary to dispose the detectors such that no difference occurs in the number of charged particles detected by the first detector  13  and the second detector  16 . When there is a concern that a signal may not be received by the second detector, a plurality of detectors are disposed around the reflection plate and the signals are added up, and thereby the signals can be regarded as the detection signals of the second detector  16 . 
     In the scanning electron microscope  101  according to the first embodiment, the upper detector  23  detects backscattered electrons at large elevation angles. As a result, it is possible to detect signal charged particles in a wide range of elevation angles from a small elevation angle to a large elevation angle of around 90°. 
     Second Embodiment 
       FIG. 6  is a configuration diagram of the scanning electron microscope  101  according to the second embodiment. In the second embodiment, in addition to the configuration described in the first embodiment, a first detector  13 - 2  and a second detector  16 - 2  are also disposed between the objective lens  8  and the sample  10 . The first detector  13  and the second detector  16  in the first embodiment are denoted as a first detector  13 - 1  and a second detector  16 - 1  (corresponding to a “third detector” and a “fourth detector” in the claims) respectively, and are disposed at the same position as in the first embodiment or at a position closer to the electric field emission cathode  1  than in the first embodiment. 
     The first detector  13 - 2  and the second detector  16 - 2  mainly detect the signal electrons  12  at small elevation angles. The signal electrons  12  at large elevation angles are close to a trajectory of the primary electron beam  3  and pass through an inside of the objective lens  8 , while the signal electrons  12  at small elevation angles collide with a lower surface of the objective lens  8  and structures around the lower surface. 
     Therefore, by disposing a detector below the objective lens  8 , it is possible to detect the signal electrons  12  at small elevation angles. 
     Obtaining an image of backscattered electrons at small elevation angles is important for measuring a structure with a low aspect ratio. An observation image thereof can be obtained using backscattered electrons detected by the first detector  13 - 2  and the second detector  16 - 2 . 
     The first detector  13 - 1  and the second detector  16 - 1  disposed at an upper portion can detect the signal electrons  12  at large elevation angles. Thus, the backscattered electrons and secondary electrons detected by these detectors can be distinguished and measured in each range of elevation angles, and these electrons can also be synthesized. In the present embodiment, since electrons at large elevation angles are detected by the first detector  13 - 1  and the second detector  16 - 1  instead of the upper detector  23 , the upper detector  23  is not necessary. 
     Third Embodiment 
       FIG. 7  is a schematic diagram showing a configuration of a detection system provided in the scanning electron microscope  101  according to the third embodiment of the disclosure. In  FIG. 7 , the first detector  13  is divided into four detectors ( 13 - 1 ,  13 - 2 ,  13 - 3 ,  13 - 4 ). That is, the first detector  13  has four detection regions. In  FIG. 7 , when the sample  10  with a trench pattern is irradiated with the primary electron beam  3 , signal electrons  12 - 1 ,  12 - 2 ,  12 - 3 , and  12 - 4  are detected in the respective detection regions marked with a cross. Other configurations are the same as those of the first and second embodiments. 
       FIG. 8  is a plan view showing an arrangement of the second detectors  16  in the third embodiment. In the present embodiment, the second detectors  16  ( 16 - 1 ,  16 - 2 ,  16 - 3 ,  16 - 4 ) are disposed for the respective four detection regions in the first detector  13 . In the following description, it is assumed that the trench pattern extends along a longitudinal direction of  FIG. 8 . 
     By obtaining an image using signals of the first detectors  13 - 1  and  13 - 3  and the second detectors  16 - 1  and  16 - 3  in a direction parallel to the direction along which a trench extends, a ratio of a signal amount of backscattered electrons can be increased. On the other hand, since secondary electrons have low kinetic energy, a trajectory is bent under an influence of charging of the sample  10 . Therefore, it is difficult for signal electrons generated at a bottom surface of the trench regardless of directions to reach the detectors. In a direction orthogonal to the trench, a part of the backscattered electrons are blocked by inner walls of the trench. As described above, by selectively acquiring signals of the detectors in the direction parallel to the trench and obtaining an image, detection efficiency with respect to the trench pattern is improved. When the trench pattern extends along a horizontal direction in  FIG. 8 , an image may be obtained in the same manner using signals of first detectors  13 - 2  and  13 - 4  and second detectors  16 - 2  and  16 - 4 . It is not always necessary to use only the detectors along the trench pattern, and at least a signal weight of the detectors along the trench pattern may be larger than a signal weight of the detectors orthogonal to the trench pattern. 
     When the first detector  13  is divided into a plurality of detectors, the tertiary electrons  15  generated from the first detector  13  that are not originally assumed may be mixed into the second detector  16 . For example, it is assumed that the second detector  16 - 1  detects the tertiary electrons  15  generated on a detection surface of the first detector  13 - 1 , and the tertiary electrons  15  generated in the first detector  13 - 2  are considered to be mixed. In order to prevent such mixing, partitions  13 - 5  for blocking the tertiary electrons  15  may be provided on division surfaces (boundary portions between the detection regions) of the first detector  13 . Alternatively, physical distances may be provided between the detectors so that the tertiary electrons  15  are not mixed. 
       FIG. 9  shows an example of a screen of a user interface presented by the scanning electron microscope  101 . The user interface can be presented on a display device such as a display included in the input and output unit  104 . 
     In a signal selection screen  201 , a user can specify coefficient values to be used in Equations  1  and  2 . Further, the user can specify a calculation condition stored in advance. A coefficient value  202  of the first detector  13  and a coefficient value  203  of the second detector  16  are displayed. When a coefficient value is a negative value, it means that a subtraction processing is performed on a signal. 
     The coefficient value  202  corresponds to −α 2 /(α 1 β 2 −α 2 β 1 ) in Equation 1 or β 2 /(α 1 β 2 −α 2 β 1 ) in Equation 2. An observation image can also be acquired based on specified values by directly inputting these values to the coefficient value  202 . Similarly, the coefficient value  203  corresponds to α 1 /(α 1 β 2 −α 2 β 1 ) in Equation 1 or β 1 /(α 1 β 2 −α 2 β 1 ) in Equation 2. An observation image can also be acquired based on specified values by directly inputting these values to the coefficient value  203 . 
     A signal calculation condition selection box  204  includes a backscattered electron signal button  205 , a secondary electron signal button  206 , a first detector detection signal button  207 , and a second detector detection signal button  208 . 
     When the backscattered electron signal button  205  is pressed, coefficient values stored in advance in Equation 1 are automatically input to the coefficient values  202  and  203 . Similarly, when the secondary electron signal button  206  is pressed, coefficient values stored in advance in Equation 2 are automatically input to the coefficient values  202  and  203 . 
     When the first detector detection signal button  207  is pressed, 1 is input to the coefficient value  202  of a first detector detection signal while 0 is input to the coefficient value  203  of a second detector detection signal, so that only a detection signal of the first detector  13  is output. Similarly, only a detection signal of the second detector  16  is output when the second detector detection signal button  208  is pressed. Further, by manually inputting the coefficient value  202  of the first detector detection signal and the coefficient value  203  of the second detector detection signal, calculation can be performed by any synthesis equation. 
     A signal processed by a predetermined calculation method is displayed on an image unit  212  of a signal image display unit  211 . When a save button  213  is pressed, image information can be saved in the storage unit  105 . When the image is stored, position information on a sample of an image and environmental information such as conditions of an applied voltage to each electrode can also be acquired and stored. 
     &lt;Modification of the Disclosure&gt; 
     In the above embodiments, the signal processing unit  103  may also acquire a shape of a sample in a height direction (e.g., a depth of a trench pattern) using an observation image of the sample, and may present a result on the user interface. 
     In the third embodiment, the partitions  13 - 5  can be obtained by, for example, disposing members such as partition plates between the detection regions of the first detector  13 , or by disposing materials different from that of the detectors at boundary portions between the detection regions. Alternatively, the partitions may be formed by other appropriate methods. That is, any partition member may be disposed between the detection regions to prevent signal electrons from being mixed. 
     In the embodiments described above, the overall control unit  102 , the signal processing unit  103 , and the input and output unit  104  can also be hardware such as a circuit device in which these functions are installed, or can be a calculation device that executes software in which these functions are installed. The overall control unit  102 , the signal processing unit  103 , and the input and output unit  104  can also be functional units of a computer system. 
     In the above embodiments, a scanning electron microscope is described as an example of a charged particle beam device in which an observation image of a sample is obtained by two-dimensionally scanning an electron beam of a probe converged on the sample, detecting secondary electrons and backscattered electrons generated from the sample, and mapping a signal amount for each scanning position. The invention can also be applied to other charged particle beam devices that obtain an observation image of a sample by irradiating the sample with a charged particle beam. 
     REFERENCE SIGN LIST 
       1  electric field emission cathode 
       2  extraction electrode 
       3  primary electron beam 
       4  condenser lens 
       5  upper scanning deflector 
       6  lower scanning deflector 
       7  objective aperture 
       8  objective lens 
       10  sample 
       11  post-stage acceleration cylinder 
       12  signal electron 
       13  first detector 
       14  reflection plate 
       15  tertiary electron 
       16  second detector 
       17  sample stage 
       18  first light 
       19  second light 
       20  Wien filter deflector 
       21  deflection electrode 
       22  deflection coil 
       23  upper detector 
       51  cathode voltage 
       52  extracting voltage 
       53  retarding voltage 
       54  post-stage acceleration voltage 
       55  voltage applied to detection surface of first detector 
       56  voltage applied to detection surface of second detector 
       101  scanning electron microscope 
       102  overall control unit 
       103  signal processor 
       104  input and output unit 
       105  storage unit