Patent Publication Number: US-11043358-B2

Title: Measuring apparatus and method of setting observation condition

Description:
INCORPORATION BY REFERENCE 
     This application claims the priority of Japanese Patent Application No. 2017-31694 filed on Feb. 23, 2017, which is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to a measuring apparatus for observing a sample form using a charged particle beam. In particular, the present invention relates to an electron microscope. 
     BACKGROUND ART 
     In the electronics field, the size of devices such as semiconductors tends to be miniaturized year by year, adding an importance on obtaining internal information derived not only from the surface of the semiconductor but also from the bottom structure such as a diffusion phase. 
     In one example, a method of observing the semiconductor surface uses a scanning electron microscope. In the following description, a scanning electron microscope is also referred to as an SEM. 
     In the observation method mentioned above, the SEM scans the sample with a primary electron beam, and detects emitted electrons (Auger electrons, secondary electrons, reflected electrons, and the like) emitted from the sample by a detector. A detection signal in the emitting direction corresponding to the emitted electrons detected by the detector is sampled at a constant cycle. Sampling the signal of the emitted electrons is performed in synchronization with a scanning signal, and an extracted signal corresponding to a pixel in the two-dimensional image is obtained. The SEM converts the intensity of the extracted signal into brightness to generate an image, or generates an image from the relationship between coordinates and brightness under scanning with the primary electron beam. 
     The use of the SEM enables acquisition of an image with high spatial resolution only by adjusting focus and astigmatism, so that the SEM is used for observation of a minute shape of a sample surface, local composition analysis, and the like. In observing the sample with the SEM, the image quality can be improved by integrating extracted signals obtained by scanning the same location a plurality of times with the electron beam. 
     Recently, soft materials, such as organic materials and biomaterials, and samples such as composite materials are subjected to the observation using the SEM. When the soft materials or the composite materials are observed, the surface is easily charged by irradiating the surface with the electron beam, causing a problem of image drift and the sample damage during the observation. Therefore, the observation with a smaller irradiation amount of the electron beam is required. On the other hand, the techniques described in PTL 1 and PTL 2 are known. 
     PTL 1 discloses that “when using the electron beam to observe the structure of the sample and evaluate the characteristic of a material, the electron beam is directed intermittently, and a secondary electron signal reflecting necessary sample information is selected by the detection time in the transient response of secondary electrons obtained under intermittent electron beam irradiation, thus preventing superimposing of unnecessary information and achieving high-quality observation.” Further, PTL 2 discloses that “steps of irradiating a fixed position in the observation region with a pulse-like intermittent electron beam, detecting a change of emitted electrons from a sample over time by the intermittent electron beam, and setting an observation condition of an electron microscope according to the change of the emitted electrons with time are included.” 
     CITATION LIST 
     Patent Literature 
     PTL 1: JP 2012-252913 A 
     PTL 2: JP 2013-214467 A 
     SUMMARY OF INVENTION 
     Technical Problem 
     In recent years, in addition to the miniaturization of semiconductors, the determination of conduction and non-conduction of semiconductors, the inspection of the lower layer capacitance, and the shape, etc. become important as the structure becomes more complicated with the transition from two-dimensional to three-dimensional structure. 
     However, as the size of semiconductors has become smaller and the structure has become more complicated, the difference of the electrical characteristic in the diffusion layer has become smaller, so that charging becomes steady. Therefore, in the conventional potential contrast method, the difference of the charge amount becomes small and the inspection sensitivity becomes low, and it becomes extremely difficult to make an inter-plug gray scale. Further, in the case of a semiconductor having a complicated structure, an area in the vicinity of the plug to be observed receives a strong influence of surface charging in the conventional method, so that the potential contrast caused by the charging is superimposed on the SEM image, thus decreasing contrast difference. Further, when a small number of electrons is present for irradiation in the same pixel, an unnecessary signal to which no electrons are directed is sent, even when the accumulation number is increased, so that the image quality is not improved. 
     Solution to Problem 
     A representative example of the invention disclosed in the present application is described below. A measuring apparatus that irradiates a sample with a charged particle beam to observe the sample includes a particle source that outputs the charged particle beam, a lens that collects the charged particle beam, a detector that detects a signal of emitted electrons emitted from the sample which is irradiated with the charged particle beam, and a control device that controls the output of the charged particle beam and the detection of the signal of the emitted electrons in accordance with an observation condition, the control device sets a first parameter for controlling an irradiation cycle of the charged particle beam, as the observation condition, a second parameter for controlling a pulse width of a pulsed charged particle beam, as the observation condition, and a third parameter for controlling detection timing of the signal of the emitted electrons within the irradiation time of the pulsed charged particle beam, as the observation condition, and the third parameter is determined in accordance with a difference in intensity of signals of the plurality of emitted electrons emitted from the irradiation position of the charged particle beam. 
     Advantageous Effects of Invention 
     According to the present invention, a highly accurate potential contrast image of a sample can be generated by controlling the measuring apparatus in accordance with observation conditions including the first parameter, the second parameter, and the third parameter. Other problems, structures, and effects that are not described above will be apparent from the following description of the embodiment. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  illustrates an example structure of a scanning electron microscope according to an embodiment. 
         FIG. 2  illustrates an example of a sample observed using a scanning electron microscope of the first embodiment. 
         FIG. 3  is an explanatory diagram for explaining a control method of the scanning electron microscope of the first embodiment. 
         FIG. 4  is an explanatory diagram for explaining the control method of the scanning electron microscope of the first embodiment. 
         FIG. 5  is an explanatory diagram for explaining the principle of a dynamic potential contrast method of the first embodiment. 
         FIG. 6  illustrates an example of an operation screen displayed on an output device of the first embodiment. 
         FIG. 7  is a flowchart illustrating processing executed by the scanning electron microscope of the first embodiment to set observation conditions. 
         FIG. 8  illustrates example images generated by the scanning electron microscope of the first embodiment. 
         FIG. 9  illustrates a model used in simulation of a second embodiment. 
         FIG. 10  illustrates an example of an operation screen displayed on the output device according to the second embodiment. 
         FIG. 11  is a flowchart illustrating processing executed by the scanning electron microscope of the second embodiment to set observation conditions. 
         FIG. 12  illustrates an example structure of a scanning electron microscope according to a third embodiment. 
         FIG. 13  illustrates an example of an operation screen displayed on the output device of the third embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. The attached drawings illustrate specific embodiments in accordance with the principle of the present invention, and provided for understanding the present invention, not for limiting interpretation of the present invention. 
     First Embodiment 
     In a first embodiment, an apparatus for generating an image (potential contrast image) on the basis of optimal observation conditions for observing a sample, and a method of setting the observation conditions will be described. 
       FIG. 1  illustrates an example structure of a scanning electron microscope  10  of the first embodiment.  FIG. 2  illustrates an example of a sample to be observed using the scanning electron microscope  10  of the first embodiment. 
     In the first embodiment, the scanning electron microscope  10  is used as an exemplary measuring apparatus used for observing the sample. Alternatively, an electron microscope using an intermittent electron beam may be used. 
     The scanning electron microscope  10  includes an electronic optical system, a stage mechanism system, an SEM control system, a signal processing system, and an SEM operation system. More specifically, the scanning electron microscope  10  includes a lens barrel  101  of an electron optical system including an electron optical system and a stage mechanism system, and a control unit  102  including an SEM control system, a signal processing system, and an SEM operation system. 
     The electron optical system includes an electron gun  111 , a deflector  113 , an objective lens  115 , and a detector  119 . The electron gun  111  outputs a primary electron beam  112 . In the present embodiment, a pulsed electron beam is emitted to a sample  116  as the primary electron beam  112 . The pulsed electron beam may be output by controlling the deflector  113  corresponding to the pulse deflector or using an electron gun  111  capable of outputting a pulsed electron beam. 
     It is assumed that the number of electrons output from the electron gun  111  can be adjusted in the range of 1 to 10000, and that the incident energy can be adjusted in the range of 1 eV to 3000 eV. 
     The primary electron beam  112  is adjusted in its focusing or the like when passing through the deflector  113  and the objective lens  115 . The primary electron beam  112  has its orbit deflected when passing through the deflector  113  and scans the sample  116  in two dimensions. The emitted electrons emitted from the sample  116  which is irradiated with the primary electron beam  112  are detected by the detector  119 . Signals of the emitted electrons detected by the detector  119  are processed by the control unit  102 . A two-dimensional image corresponding to the irradiation position of the primary electron beam  112  is displayed on the output device  125 . 
     A stage mechanism system includes a sample holder  117  provided with a stage for placing the sample  116 . The stage is subjected to tilt control and movement control in three-dimensional directions (XYZ axes). The sample  116  is assumed to be a semiconductor substrate  200  as illustrated in  FIG. 2 . The semiconductor substrate  200  includes an insulating film  201 , a contact plug  202 , a gate  203 , and the like. The semiconductor substrate  200  illustrated in  FIG. 2  is an example and is not limited thereto. 
     The control unit  102  includes a calculation device  121 , a storage device  122 , a pulsed electron control device  123 , an input device  124 , and an output device  125 . The control unit  102  may include a storage medium such as a hard disk drive (HDD) and a solid state drive (SSD). 
     The calculation device  121  executes predetermined calculation processing in accordance with a program stored in the storage device  122 . The calculation device  121  may be, for example, a central processing unit (CPU) or a graphics processing unit (GPU). 
     The storage device  122  stores a program executed by the calculation device  121  and data used by the program. The storage device  122  also includes a temporary storage area such as a work area used by the program. The storage device  122  may be, for example, a memory. The programs and data stored in the storage device  122  will be described later. 
     The pulsed electronic control device  123  controls the output of the pulsed electron beam. The pulsed electronic control device  123  of the present embodiment is connected in a communicable manner to the deflector  113 . 
     The input device  124  is a device for inputting data, and includes a keyboard, a mouse, a touch panel, and the like. The output device  125  is a device that outputs data, and includes a touch panel, a display, and the like. 
     The storage device  122  stores a program for realizing the control module  131  and the image generation module  132 . Further, the storage device  122  stores condition information  133  and image information  134 . The storage device  122  may store programs and information (not illustrated). 
     A control module  131  controls constituent components in the lens barrel  101  of the electron optical system. The image generation module  132  generates an image from the signals of the emitted electrons. In the present embodiment, the control module  131  or the image generation module  132  samples the signals of the emitted electrons. 
     Condition information  133  is information for controlling observation conditions. The observation conditions are generated as data (entry) associated with an acceleration voltage, an irradiation current, a pulse width (irradiation time), a pixel split number (irradiation cycle), a timing delay, a pointer indicating storage position of image, and so on, and registered in the condition information  133 . The entry may include identification information, type, and the like of the sample  116 . Image information  134  is information for managing the generated potential contrast image. 
     In the present embodiment, the SEM control system includes the control module  131  and the pulsed electron control device  123 , the signal processing system includes the image generation module  132 , and the SEM operation system includes the input device  124  and the output device  125 . 
       FIGS. 3 and 4  are explanatory diagrams for explaining a control method of the scanning electron microscope  10  of the first embodiment.  FIG. 3  illustrates scanning control using the pulsed electron beam, and  FIG. 4  illustrates sampling control of the scanning electron microscope  10 . 
     A rectangle  300  indicates the irradiation range of the primary electron beam  112 . In  FIG. 3 , the scanning electron microscope  10  starts scanning from the upper left of the rectangle  300  to the lower left of the rectangle  300  with the primary electron beam  112 . The specific scanning trajectory of the primary electron beam is as illustrated by arrows in  FIG. 3 . Movement in the X direction during the scanning control is based on a control signal to the deflector  113 . 
     The pulsed electron control device  123  cooperates with the control module  131  to control the scanning with the pulsed electron beam, and the image generation module  132  controls the detection timing of the emitted electrons. It is possible to achieve the scan control synchronized with the irradiation of the pulsed electron beam, and acquire a potential contrast image from the signals of the emitted electrons detected in synchronization with the irradiation of the pulsed electron beam. 
     A conventional scanning electron microscope irradiates the sample  116  with the primary electron beam  112  by adjusting a scanning speed and a probe current to control an electron irradiation density (scanning line density) of the primary electron beam  112  emitted to the sample  116  in a single scan. 
     Electrons emitted from the sample  116  are strongly affected by the surface charge in the vicinity of the observation target. When the interaction between the emitted electrons and the charge is weak, a potential contrast image having a smaller influence of the charge is obtained, but when the interaction between the emitted electrons and the charge is strong, the potential contrast due to the charge is superimposed on the SEM image. Further, the interaction between the emitted electrons and the charge strongly depends on the electrical characteristic of the sample  116 . 
     The scanning electron microscope  10  of the present embodiment irradiates the sample  116  with the pulsed electron beam in accordance with pixel split control to solve the problem mentioned above. Accordingly, the charge of the sample  116  can be controlled, and the transient state of charging of the sample  116  can be visualized. 
     In the pixel split control, the scanning line density, the time interval of irradiation of the pulsed electron beam, and the pulse width of the pulsed electron beam are controlled. More specifically, in addition to the control of the scanning speed and the scanning line density for adjusting the probe current, the time interval (irradiation cycle) and the pulse width of irradiation of the pulsed electron beam are controlled for each pixel in the scanning direction (X direction). The charge of the sample  116  can be highly controlled when the control in the Y direction is combined. 
       FIG. 3  is a graph of the polarizer control signal and the electron beam irradiation control signal, where the horizontal axis represent time and the vertical axis represents a signal intensity. As the intensity of the polarizer control signal increases, the irradiation position of the primary electron beam  112  moves in the X direction. Further, in accordance with the electron beam irradiation control signal, the sample  116  is irradiated with the pulsed electron beam at constant time intervals from the left end to the right end. When the scanning electron microscope  10  reaches the right end, that is, when the intensity of the polarizer control signal changes to the initial value, the scanning electron microscope  10  moves to the scanning line spaced by a predetermined distance in the Y direction to irradiate the sample  116  from the left end to the right end. If the downward movement along the Y direction is not allowed, the scanning electron microscope  10  moves to the uppermost scanning line among unprocessed scanning lines and performs similar processing. 
     In order to obtain a potential contrast image according to the difference in the electrical characteristics of the sample, the present control is effective as it being capable of adjusting the interaction between the charge in the vicinity of the observation object and the emitted electrons. 
     The scanning electron microscope  10  of the present embodiment adopts pixel split control, and further implements a dynamic potential contrast method to obtain a highly accurate potential contrast image. 
     The reference signal illustrated in  FIG. 4  is a signal serving as an operation reference of the SEM control system and the signal processing system. The control unit  102  of the present embodiment performs control using a control signal for signal detection sampling, in addition to the control using the polarizer control signal and the primary electron beam irradiation control signal described above. The control signals are controlled to synchronize with the reference signal. The control unit  102  can adjust each control signal with an accuracy of 1/10 of the time resolution of the reference signal. 
     Time for which the pulsed electron beam per pixel in the TV scanning lines stays on the sample  116  is equal to irradiation time, that is, a pulse width Tp, of the pulsed electron beam. The detection signal of the emitted electrons indicates a signal of the detected emitted electrons. As illustrated in  FIG. 4 , the intensity of the signal decreases with time. 
     The image generation module  132  performs sampling so as to detect emitted electrons once at any timing during irradiation of the pulsed electron beam. The detection timing is adjusted using a timing delay Td that indicates a delay time from the start of irradiation of the pulsed electron beam. 
     In the present embodiment, the control condition regarding the output of the pulsed electron beam and the control condition regarding the detection timing of the signal of the emitted electron are set as the observation condition. In the following, a control condition regarding irradiation of the pulsed electron beam is described as the scanning condition, and a control condition regarding detection timing of the signal of the emitted electrons is also described as a detection condition. 
     In the present embodiment, the detection timing is determined to be in the range of 10 MHz to 1000 MHz. 
     Here, a reason for obtaining a highly accurate potential contrast image by adjusting the detection timing of the signal of emitted electrons is described below with reference to  FIG. 5 . 
       FIG. 5  is a diagram for explaining the principle of the dynamic potential contrast method of the first embodiment. 
     When the sample  116  as illustrated in  FIG. 2  is irradiated with a pulsed electron beam, the emitted electrons emitted from the adjacent contact plug  202  are detected. The emitted electron beam changes as illustrated in  FIG. 5 . Here, the horizontal axis of  FIG. 5  indicates time, and the vertical axis indicates the intensity of the signals of the emitted electrons such as current. 
     When the image generation module  132  detects the signal of the emitted electron at detection timing  1 , the intensity of the signal of the emitted electrons emitted from the adjacent contact plug  202  is substantially the same intensity, so that the potential contrast image having no contrast difference is generated. On the other hand, when the image generation module  132  detects the signal of the emitted electrons at detection timing  2 , the intensity of the signal of the emitted electrons emitted from the adjacent contact plug  202  is different, so that the potential contrast image including the contrast difference is generated. It is difficult to obtain a highly accurate potential contrast image when the detection timing is set at a later point along the time axis, because the intensity of the signal of the emitted electrons, which is emitted along with charging of the sample  116 , decreases with time. The temporal change of the intensity of the signal of the emitted electrons mainly depends on the pulse width. 
     In the present embodiment, the detection timing is adjusted so as to obtain an optimal potential contrast image in consideration of the temporal change (transient characteristic) of the emitted electrons as described above. 
     Next, a method of setting observation conditions is described using  FIGS. 6 and 7 . 
       FIG. 6  illustrates an example of an operation screen  600  displayed on the output device  125  according to the first embodiment. 
     The operation screen  600  is a screen which appears during setting of the observation condition, and includes a condition setting button  601 , a condition setting area  602 , a transient characteristic acquisition button  603 , a transient characteristic display area  604 , an image acquisition button  605 , an image display area  606 , and save buttons  607  and  608 . 
     The condition setting button  601  is an operation button for setting the value set in the condition setting area  602  as a parameter to be included in the observation condition. When the condition setting button  601  is operated, the control unit  102  temporarily stores, in the storage device  122 , the observation condition including the value set in the condition setting area  602 . 
     The condition setting area  602  is an area for setting an observation condition. The condition setting area  602  includes an acceleration voltage field, an irradiation current field, a pulse width field, a pixel split number field, a timing delay field, and a detection start time field. 
     The acceleration voltage field, the irradiation current field, the pulse width field, and the pixel split number field are fields for inputting parameters to be set as scanning conditions. The pulse width field is for a parameter that specifies the pulse width of the pulsed electron beam, that is, the time for which the electron beam continues to be applied to the sample  116 , that is, the irradiation time. The pixel split number field is for a parameter that specifies the number of pixels specifying the irradiation position, that is, the irradiation cycle of the pulsed electron beam. 
     The timing delay field and the detection start time field are fields for inputting parameters to be set as detection conditions. Here, the timing delay field is a field for specifying a parameter for determining the detection timing of the emitted electrons. The detection start time field is a field for specifying an initial value of timing delay when a plurality of potential contrast images are acquired to determine the timing delay. 
     When the transient characteristic acquisition button  603  is operated, no value may be set in the timing delay field and the detection start time field. 
     The transient characteristic acquisition button  603  is an operation button for acquiring a graph illustrating a temporal change of the intensity of the signal of the emitted electron. The scanning electron microscope  10  of the present embodiment, upon receipt of the operation of the transient characteristic acquisition button  603 , irradiates the sample  116  with the pulsed electron beam in accordance with the observation conditions, and stores data indicating the temporal change of the signal of the emitted electrons in the storage device  122 . The transient characteristic display area  604  is an area for displaying a graph illustrating the temporal change of the intensity of the signal of the emitted electrons. 
     The image acquisition button  605  is an operation button for giving an instruction on generation of the potential contrast image. The image display area  606  is an area for displaying the potential contrast image generated by sampling signals of the emitted electrons detected in accordance with the designated timing delay. 
     The save buttons  607  and  608  are operation buttons for registering the set observation conditions in the condition information. When the save button  607  is operated, the control unit  102  registers, in the condition information  133 , the observation conditions used when generating all the potential contrast images displayed in the image display area  606 . When the save button  608  is operated, the control unit  102  registers, in the condition information  133 , the observation condition used when generating the potential contrast image selected from the image display area  606 . 
     The user can set observation conditions including optimal parameters without trial and error by referring to the graph of the temporal change of the signal of the emitted electrons and the potential contrast image. 
       FIG. 7  is a flowchart for explaining the processing executed when the scanning electron microscope  10  of the first embodiment sets the observation conditions. 
     When the transient characteristic acquisition button  603  is operated, the scanning electron microscope  10  starts processing described below. 
     The control unit  102  sets scanning conditions in the lens barrel  101  of the electron optical system (step S 101 ). 
     Specifically, the control module  131  outputs a setting instruction including scanning conditions to the pulsed electron control device  123 . As a result, the field of view, an acceleration voltage, and the number of pixel splits are set in the lens barrel  101  of the electron optical system. 
     The control unit  102  instructs the lens barrel  101  of the electron optical system to irradiate the sample  116  with the pulsed electron beam (step S 102 ). 
     Upon receipt of the instruction, the lens barrel  101  of the electron optical system periodically irradiates the sample  116  with the pulsed electron beam having a predetermined pulse width on the basis of the scanning conditions set by the pulsed electron control device  123 . 
     At this time, the control module  131  records, in the storage device  122 , data indicating the temporal change of the signal of the emitted electrons detected by the detector  119 . After the recording is completed, the control unit  102  outputs, to the output device  125 , a message prompting the user to operate the transient characteristic acquisition button  603 . 
     The control unit  102  displays a graph illustrating the temporal change of the intensity of the signal of the emitted electrons, and receives the setting of the timing delay (step S 103 ). 
     Specifically, when accepting the operation of the transient characteristic acquisition button  603 , the control module  131  generates a graph from the data recorded in the storage device  122 , and displays the generated graph in the transient characteristic display area  604 . Alternatively, this may be set automatically by the control module  131 . For example, the control module  131  determines the detection timing at which the difference between the plurality of signals is maximum on the basis of the temporal change of the signal of the emitted electrons, and sets the timing delay corresponding to the detection timing. 
     The user sets a value in the timing delay field or the detection start time field of the condition setting area  602  according to the graph. The user also operates the image acquisition button  605 . 
     The control unit  102  samples the signal of the emitted electrons on the basis of the set timing delay and, when receiving the operation of the image acquisition button  605 , generates the potential contrast image (step S 104 ). 
     Specifically, the image generation module  132  samples the signal of the emitted electrons detected by the detector  119  on the basis of the set timing delay, and generates the potential contrast image using the sampled signal of the emitted electrons. In other words, the image generation module  132  refers to the graph of the temporal change of the signal of the emitted electrons, and obtains the value of the time corresponding to the detection timing. 
     The image generation module  132  also outputs the potential contrast image to the image display area  606  of the operation screen  600  displayed on the output device  125 . The image generation module  132  outputs the potential contrast image and the time delay in association with each other. 
     The user refers to the displayed potential contrast image and registers the observation conditions corresponding to the optimal potential contrast image in the condition information  133 . When the save button  607  or  608  is operated, the control module  131  sets the time delay associated with the selected potential contrast image as the detection condition, and generates the observation conditions including the scanning condition set in step S 101 . The control module  131  registers the observation conditions in the condition information  133 . 
     Note that the method of setting the time delay in the observation condition and the method of registering the observation condition in the condition information  133  are only examples, and the present invention is not limited to these methods. 
     When a value is set in the detection start time field, the image generation module  132  uses this value as a reference value and sets a plurality of candidate timing delays (detection timings). Further, the image generation module  132  generates a potential contrast image corresponding to each detection timing. For example, the image generation module  132  uses the value in the detection start time field as a reference value, and sets the candidate timing delay by shifting it to about 20 ns. When a value is set in the timing delay field, the control unit  102  sets a detection timing corresponding to the uniquely determined timing delay, and generates a potential contrast image corresponding to the detection timing. 
     In the present embodiment, the user refers to the image and registers the observation condition, but the control module  131  may automatically set the timing delay. For example, in the case of the graph illustrated in  FIG. 5 , the control module  131  sets the time at which the difference between the intensities of the two signals of the emitted electrons is maximum as the detection timing. In this case, the processing described above is performed instead of the processing of steps S 103  and S 104 . When three or more signals of the emitted electrons are present, the control module  131  sets the time at which the sum of the differences in intensity of the respective signals is minimum as the detection timing. 
     After completion of setting the observation conditions, the sample  116  is actually observed. When receiving the observation request of the sample  116 , the control unit  102  of the present embodiment acquires the observation condition from the condition information, and controls the output of the pulsed electron beam and the detection of the signal of the emitted electrons on the basis of the observation conditions. Specifically, the pulsed electron beam is emitted to the sample  116  on the basis of the scanning conditions including parameters such as the scanning line density, the time interval of irradiation of the pulsed electron beam to the sample  116 , and the pulse width. Meanwhile, the signal of the emitted electrons is sampled on the basis of the detection conditions including the timing delay, and the image is generated using the sampled signals. 
       FIG. 8  is a view illustrating example images generated by the scanning electron microscope  10  of the first embodiment. 
     An image  1  is a potential contrast image when the timing delay is small. The image  1  is, for example, an image generated from the signal of the emitted electrons detected at the detection timing  1  of  FIG. 5 . An image  2  is a potential contrast image when there is a long timing delay. The image  2  is, for example, an image generated from the signal of the emitted electrons detected at the detection timing  2  of  FIG. 5 . It is possible to examine the structure of the sample  116  with high accuracy in the image  2 , as the contrast of the image  2  is clearer than the contrast of the image  1 . 
     According to the first embodiment, the scanning electron microscope  10  can generate a high precision potential contrast image of the sample  116  having a fine structure. 
     Second Embodiment 
     A second embodiment differs from the first embodiment in that the observation conditions are set on the basis of simulation using design data of the sample  116 . The second embodiment will be described below by focusing on the difference from the first embodiment. 
     The structure of the scanning electron microscope  10  of the second embodiment is the same as the structure of the scanning electron microscope  10  of the first embodiment, and the description thereof is not repeated. Further, the control method of the scanning electron microscope  10  on the basis of the dynamic potential contrast method to be used is the same as the control method of the first embodiment, and the description thereof is not repeated. 
     In the second embodiment, it is assumed that design data (RC constant) of the sample  116  is known in advance, and electrical characteristics of the sample  116  are modeled using an equivalent circuit. 
       FIG. 9  illustrates a model used for the simulation of the second embodiment. 
     In the second embodiment, as illustrated in  FIG. 9 , a potential contrast image is simulated by the dynamic potential contrast method using an energy distribution model of the emitted electrons in which a potential saddle point generated by the surface potential due to charging is present. 
     The left side of  FIG. 9  illustrates a distribution of the electric potential, and the right side of  FIG. 9  is a graph illustrating the change of the electric potential. 
     In the model illustrated in  FIG. 9 , when the sample  116  is irradiated with the pulsed electron beam, a surface potential V s  is generated depending on the electrical characteristics of the sample  116 , and a potential saddle point V φ  is generated from the interaction of the electrolysis in the Z direction and the surface potential of the sample in the surface of the sample  116 . The potential saddle point has a negative potential with respect to the surface potential, and acts as an energy barrier V b  relative to the emitted electrons. The energy barrier is given as the difference between the potential saddle point and the surface potential. As the surface potential increases, the emission current (signal intensity) of the emitted electrons decreases and the chargeability decreases. 
     In the present embodiment, the integral value of the energy distribution N(W) of the emitted electrons in the model illustrated in  FIG. 9  is set as illustrated in equation (1). 
     
       
         
           
             
               
                 
                   
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     where δ represents an emission rate of the emitted electrons, I p  represents an irradiation current, and β represents a characteristic parameter of the energy distribution of the emitted electrons. For example, if the sample  116  is a metal, β is set to 8, and if the sample  116  is an insulator, β is set to 4. 
     By using the above energy distribution model, the electrical characteristics of the sample  116  can be estimated as the equivalent circuit (RC parallel circuit). Further, when the equivalent circuit corresponding to the sample  116  is wired to the current source controlled according to the equation (1), the current source corresponds to the emission current of the emitted electrons, so that the potential contrast image can be simulated. When observing the semiconductor substrate  200  as the sample  116 , the contact plug  202  is modeled with the RC parallel circuit to simulate the temporal change of the emission current of the emitted electrons on the basis of the RC constant of the contact plug  202 . 
       FIG. 10  illustrates an example of an operation screen  1000  displayed on the output device  125  of the second embodiment. 
     The operation screen  1000  is a screen displayed at the time of setting the observation condition, and includes a condition setting button  1001 , a condition setting area  1002 , a transient characteristic calculation button  1003 , a transient characteristic display area  1004 , an image acquisition button  1005 , an image display area  1007 , and save buttons  1008  and  1009 . 
     The condition setting button  1001  is an operation button for setting the value set in the condition setting area  1002  as a parameter to be included in the simulation setting information. Here, the simulation setting information includes the observation conditions and the transient characteristic conditions. 
     The condition setting area  1002  is an area for setting the observation conditions and the transient characteristic conditions. The condition setting area  1002  includes an acceleration voltage field, an irradiation current field, an emission rate of the emitted electrons field, a pulse width field, a pixel split number field, a timing delay field, a resistance value field, a capacitance field, and a material selection field. 
     The acceleration voltage field, the irradiation current field, the pulse width field, the pixel split number field, and the timing delay field are the same as those included in the condition setting area  1002 . The emission rate of the emitted electrons, the resistance value field, the capacitance field, and the material selection field are fields for inputting values to be set as the transient characteristic conditions. 
     Note that different simulation setting information is displayed in a tab format. For each tab, different simulation setting information can be set for each type of structure (e.g., contact plug  202  or the like) of the sample  116 . 
     The transient characteristic calculation button  1003  is an operation button for giving an instruction on execution of simulation. When the transient characteristic calculation button  1003  is operated, the control module  131  performs simulation based on the simulation setting information. The control module  131  records data indicating temporal change of emission current (signal) of emitted electrons in the storage device  122  as a simulation result. 
     The transient characteristic display area  1004  is an area for displaying a graph illustrating the temporal change of the emission current of the emitted electrons. In the transient characteristic display area  1004  of the present embodiment, a graph corresponding to each piece of simulation setting information is displayed. Note that, by switching the tab, a graph of different simulation setting information can be referred to. When the “select all” tab is selected, a graph compiling graphs of each piece of simulation setting information is displayed. 
     The image acquisition button  1006  is an operation button for giving an instruction on generation of a potential contrast image using a simulation result. In the present embodiment, the potential contrast image is generated on the basis of the simulation result and the timing delay corresponding to the graph displayed in the transient characteristic display area  1004 . 
     At this time, the control module  131  performs sampling at the detection timing corresponding to the value set in the timing delay field to generate the potential contrast image. The control module  131  may set a plurality of detection timings using the value in the timing delay field as the reference value, and may generate the potential contrast images corresponding to the individual detection timings. 
     The image display area  1007  is an area for displaying the generated potential contrast image. The save buttons  1008  and  1009  are operation buttons for registering the observation conditions included in the simulation setting information in the condition information  133 . 
       FIG. 11  is a flowchart for explaining processing executed when the scanning electron microscope  10  of the second embodiment sets the observation conditions. 
     The scanning electron microscope  10  starts the processing described below when the transient characteristic calculation button  1003  is operated. 
     The control unit  102  sets parameters included in the simulation setting information stored in the storage device  122  in the simulation model (step S 201 ). Here, parameters included in the scanning conditions and the transient characteristic conditions are set. 
     The control unit  102  executes the simulation and displays a graph on the basis of the simulation result (step S 202 ). 
     When several pieces of simulation setting information are set, the control module  131  performs simulation about each piece of simulation setting information. In this case, the transient characteristic display area  1004  displays a graph corresponding to each piece of simulation setting information. 
     The control unit  102  receives the setting of the timing delay (step S 203 ). 
     The control unit  102  outputs a message prompting the user to input a value in the timing delay field and operate the image acquisition button  1006 . The user sets a value in the timing delay field on the basis of the graph displayed in the transient characteristic display area  1004 , and operates the image acquisition button  1006 . 
     The control unit  102  samples the signals of the emitted electrons on the basis of the set timing delay to generate the potential contrast image (step S 204 ). 
     Specifically, the control module  131  inputs, to the image generation module  132 , data indicating the temporal change of the signals of the emitted electrons and the timing delay, and gives an instruction on generation of a potential contrast image. The image generation module  132  performs signal sampling of emitted electrons using the timing delay, and generates the potential contrast image using the sampled signal of the emitted electrons. 
     The method of setting the time delay to the observation conditions and the method of registering the observation conditions in the condition information  133  are the same as the methods of the first embodiment, and the description thereof is not repeated. Further, the control method of the scanning electron microscope  10  on the basis of the observation conditions of the second embodiment is also the same as that of the first embodiment, and the description thereof is not repeated. 
     According to the second embodiment, the scanning electron microscope  10  can generate a high precision potential contrast image of the sample  116  having a fine structure. In addition, the observation conditions can be set without actually irradiating the sample  116  with the pulsed electron beam. 
     Third Embodiment 
     In a third embodiment, the structure of the scanning electron microscope  10  is partially different. The third embodiment will be described below by focusing on the difference from the first embodiment. 
       FIG. 12  illustrates an example of the structure of the scanning electron microscope  10  of the third embodiment. 
     In the scanning electron microscope  10  of the third embodiment, the structure of the lens barrel  101  of the electron optical system is partially changed to observe the sample  116  made of, for example, an organic material that is easily charged. The configuration of the control unit  102  of the third embodiment is the same as the configuration of the control unit  102  of the first embodiment, and the description thereof is not repeated. 
     The lens barrel  101  of the electron optical system of the third embodiment includes an electrode  151  between the objective lens  115  and the sample holder  117  on which the sample  116  is placed. The control unit  102  controls the electric field in the vicinity of the irradiation area of the primary electron beam  112  of the sample  116  using the electrode  151 . More specifically, the control unit  102  controls the electrode  151  so that the acceleration voltage of the primary electron beam  112  is low. As a result, the charging of the sample  116  by the primary electron beam  112  can be reduced. 
       FIG. 13  illustrates an example of an operation screen  1300  displayed on the output device  125  of the third embodiment. 
     The operation screen  1300  is a screen displayed at the time of setting the observation conditions, and includes a condition setting button  1301 , a condition setting area  1302 , a transient characteristic acquisition button  1303 , a transient characteristic display area  1304 , an image acquisition button  1305 , an image display area  1306 , and save buttons  1307  and  1308 . 
     The condition setting button  1301  and the condition setting area  1302  are the same as the condition setting button  601  and the condition setting area  602 , respectively. Note that the condition setting area  1302  includes a retarding voltage field. The retarding voltage field is a field for setting parameters for controlling the electrode  151 . 
     The transient characteristic acquisition button  1303 , the transient characteristic display area  1304 , the image acquisition button  1305 , the image display area  1306 , and the save buttons  1307  and  1308  are the same as the transient characteristic acquisition button  603 , the transient characteristic display area  604 , the image acquisition button  605 , the image display area  606 , and the save buttons  607  and  608 , respectively. 
     The method of setting the time delay to the observation conditions and the method of registering the observation conditions to the condition information  133  in the third embodiment are the same as the methods of the first embodiment, and the description thereof is not repeated. The control method of the scanning electron microscope  10  on the basis of the observation conditions of the third embodiment is also the same as the control method of the first embodiment, and the description thereof is not repeated. 
     According to the third embodiment, the scanning electron microscope  10  can generate a highly accurate potential contrast image of the sample  116  which is easily charged and has a fine structure. 
     The present invention is not limited to the above-described embodiments, and may include various modifications. For example, the embodiments described above have been given in detail to facilitate the understanding of the present invention, and are not necessarily limited to those including all constituent components described above. Further, some of the constituent components of each embodiment may be added, deleted, or substituted for by other constituent components. 
     Further, all or part of the above-described configurations, functions, processing units, processing means, and the like may be formed using hardware by, for example, integrated circuit design. The present invention can also be realized by a program code of software that implements the functions of the embodiment. In this case, a storage medium recording the program code is provided to the computer, and a processor included in the computer reads the program code stored in the storage medium. In this case, the program code itself read from the storage medium implements the functions of the above-described embodiments, and the program code itself and the storage medium storing the same constitute the present invention. As a storage medium for supplying such a program code, for example, a flexible disk, a CD-ROM, a DVD-ROM, a hard disk, a solid state drive (SSD), an optical disk, a magneto-optical disk, a CD-R, a magnetic tape, a non-volatile memory card, a ROM, or the like is used. 
     Further, the program code for providing the functions described in the present embodiment can be implemented by a wide range of programs or script languages such as, for example, assembler, C/C++, Perl, a shell, PHP, Java (registered trademark). 
     Further, by distributing the program code of the software for realizing the functions of the embodiment through a network, the program code is stored in a storage means such as a hard disk or a memory of a computer or a storage medium such as a CD-RW or CD-R. Alternatively, a processor included in the computer may read out and execute the program code stored in the storage means or the storage medium. 
     In the above-described embodiments, the control lines and the information lines have been considered to be necessary for description, but those lines do not always represent all lines required for a manufactured product. Alternatively, all constituent components may be connected mutually.