JP2008159574A - Scanning electron microscope - Google Patents

Abstract

Provided is a technique capable of performing accurate automatic focus adjustment with an image of one detector when there are a plurality of foreign matters or patterns that can be imaged only by a specific detector and they are included independently. .
A plurality of detectors for detecting a secondary signal generated from a sample by irradiating the sample with an electron beam, and arithmetic means for synthesizing signals obtained by the detector. At least two are arranged symmetrically with respect to the electron beam, and the focus of the electron beam is adjusted based on the signal of each detector or the synthesized signal.
[Selection] Figure 1

Description

  The present invention relates to a scanning electron microscope, and more particularly to a scanning electron microscope that automatically adjusts a plurality of types of images having different characteristics.

  An inspection process is indispensable for improving the yield of semiconductor devices, and an electron microscope for inspecting and identifying wafer abnormalities is no longer indispensable in order to identify the cause of yield loss and improve production efficiency. ing. In addition, for efficient inspection, it is essential to improve the inspection speed, and it is desired that many points can be inspected in a short time.

  As one of the elements, an automatic focus adjustment function for adjusting the focus of an image in an electron microscope is an indispensable function for capturing a clear image. This is a method described in Patent Document 1 shown below. Attempts have been made to perform accurate automatic adjustment such as the first. The automatic focus adjustment function described in Patent Document 1 acquires a plurality of images while changing the focal point, calculates a high-frequency response of those images, calculates a focal point with the largest response, and performs automatic adjustment. A method is described.

JP-A-2005-332593.

  In the case of a semiconductor wafer, there are various types of foreign matters and defects that cause device malfunction. For example, a pattern that is relatively easy to observe, such as the edge of a pattern, a thin or uneven foreign object such as a residue after removal of chemicals, or a thin unevenness such as a scratch generated during polishing or a thin film When forming, there are things that are difficult to observe, such as thin hemispherical irregularities that result from forming a thin film on a wafer that already contains foreign matter. Some of these devices, including microscopes that inspect foreign objects and defects, can acquire multiple images with different physical characteristics using multiple detectors, and capture images according to the characteristics of the foreign objects and defects. To be done. However, when there are a plurality of types of foreign matters and defects that can be seen only in the image of each detector, there are cases where the above-mentioned automatic focus adjustment function cannot cope.

  As an example, consider the following case. FIG. 1 is a schematic cross-sectional view of an electron microscope, and shows only the portions necessary for explanation. Reference numerals 122 to 123 in the figure are detectors, and these detectors serve to detect secondary electrons and reflected electrons, which are secondary signals generated by irradiating the wafer with electrons from the electron gun 101. . The image can be confirmed by synchronizing the signal detected by the detector with the scan of the electron beam from the electron gun and the scan signal of the monitor 117 for confirming the image.

  In general, secondary electrons generated from a sample by irradiation with an electron beam are defined as electrons having an energy of 50 eV or less, and the generation efficiency varies depending on the sample, and is expressed in black and white on the screen. Further, the corner portion such as the edge of the pattern is characterized in that the amount of secondary electrons is generated more than the portion on the other plane even with the same material. On the other hand, the reflected electrons generated from the sample by the irradiation of the electron beam are defined as electrons having an energy of 50 eV or more, and have the property of reflecting the shape of the irradiated material, and the yield varies depending on the installation direction of the detector. For example, in the shallow hole portion, the portion facing the detector appears bright, and the portion opposite to the detector appears dark. By utilizing this property, for example, by installing the detectors in the opposite direction, a pair of shadow images having different shadowing methods can be acquired. The behavior of secondary electrons and backscattered electrons generated from the sample varies depending on the physical conditions and the electric field from the sample to the detector. Therefore, it is possible to detect the secondary electrons and backscattered electrons separately with each detector. Although difficult, it is possible to meet a certain yield by arranging the detector and controlling the electric field.

  Conventionally, when performing focus adjustment, either an image generated from a secondary signal captured by a secondary electron detector or an image generated from a secondary signal captured by a backscattered electron detector is used. The focus was adjusted. However, in this case, it may be difficult to perform appropriate automatic adjustment in all scenes. For example, let us consider a case where a spot-like foreign material having no thickness is included where there is no pattern and a case where unevenness having a small height difference is included. The former is well reflected in the secondary electron image, and conversely is hardly reflected in the reflected electron image because there is no thickness. On the other hand, the unevenness appears well in the reflected electron image, but the secondary electron image does not appear well because there is no difference in material and no edge is included. For this reason, when detection is performed with an image (hereinafter referred to as a secondary electron image) by a detector that detects secondary electrons, a defect appears in an image (hereinafter referred to as a reflected electron image) detected by a detector that detects reflected electrons. In the case of an image including, automatic adjustment may not be performed correctly and the image may be out of focus. The same applies to the reverse case. When a circuit pattern or the like is included around a foreign object, the image can be adjusted with a clue, but when there is nothing around, it cannot be performed.

  Therefore, an object of the present invention is to enable accurate automatic focus adjustment with an image of one detector when there are a plurality of foreign objects or patterns that can be imaged only by a specific detector and they are included independently. Is to provide new technology.

  Further, it is to provide a technique capable of automatically adjusting an image having a low contrast such as a concavo-convex image even when the focus position is difficult to understand.

  In order to solve the above problems, an embodiment of the present invention combines a plurality of detectors that detect a secondary signal generated from a sample by irradiating the sample with an electron beam and a signal obtained by the detector. And at least two of the detectors are arranged symmetrically with respect to the electron beam, and the focus of the electron beam is adjusted based on a signal of each detector or a synthesized signal. It is a thing.

  An electron lens for converging the electron beam emitted from the electron source onto the sample and a detector for detecting a secondary signal generated from the sample by the electron beam irradiation, provided at a predetermined interval. A pair of first and second detectors, a third detector, and the focal point of the electron beam with respect to the sample are changed a plurality of times, and are detected by the first, second, and third detectors at the respective focal points. Storage means for generating and storing first, second, and third secondary signal data from the secondary signal, and synthesis for combining the first, second, and third secondary signal data at the respective focal points An evaluation value is calculated on the basis of an intensity signal obtained by performing a predetermined data conversion process on the secondary signal data at each focus subjected to the composite calculation process and the data calculation means for performing the calculation process, and corresponding to each focus Calculation based on evaluation value With the focal, it is obtained by the structure having a focus adjusting means for adjusting the focus of the electron beam on the sample.

  According to the present invention, it is possible to stably perform automatic focus adjustment even when a pattern that appears only on a specific detector or when a plurality of foreign objects are included independently, or even in an image that is difficult to adjust focus with low contrast. .

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  A scanning electron microscope shown in FIG. 1 includes an electron gun 101, a lens 102, a deflector 103, an objective lens 104, a sample stage 106, a lens control circuit 110, a deflection control circuit 111, an objective lens control circuit 112, and an analog / digital converter. 113, address control circuit 114, image memory 115, control means 116, display 117, computer 118, image processing means 119, keyboard 120, mouse 121, secondary particle detector 122, backscattered electron detector pair 123, moving stage 124 It consists of input means. In addition, 105 indicates a sample, 107 indicates an electron beam, 108 indicates a secondary electron, and 109 indicates a reflected electron. In the figure, a column for maintaining a vacuum is omitted. The backscattered electron detector pair 123 is installed at a position facing each other on a straight line in order to capture a dual shadow image, but the arrangement is not limited to the case shown in the figure.

  An electron beam 107 radiated from the electron gun 101 is converged by the lens 102, scanned and deflected two-dimensionally by the deflector 103, converged by the objective lens 104, and irradiated onto the sample 105. When the sample 105 is irradiated with the electron beam 107, secondary electrons 108 and reflected electrons 109 are generated according to the shape and material of the sample. These 108 and 109 are detected and amplified by the detector 122 or 123, and converted into a digital value by the analog / digital converter 113. The signal from the detector pair 123 is used to form an L image and an R image that are reflected electron images, and the signal from the detector 122 is used to form an S image that is a secondary electron image. The data converted into the digital value is stored in the image memory 115. As an address of the image memory 115 at this time, the address control circuit 114 generates an address synchronized with the scanning signal of the electron beam. Further, the image memory 115 transfers the stored image data of the SEM image to the computer 118 as needed. The computer 118 calculates the focus evaluation value from the image, performs function fitting to the evaluation value, and calculates the peak of the fitting function, sends a focus adjustment signal to the objective lens control circuit 112, and controls the objective lens to adjust the focus. , Etc. are processed. In 112, it is also used for focus adjustment when an image is captured while adjusting the focus position in the autofocus process. The image processing unit 119 reads the image data in the image memory 115 and executes the calculation. This image processing means may have a plurality of functions, but it needs to have at least a function for adding / subtracting images and a function for multiplying a certain value.

  A sample 105 observed with a scanning electron microscope is held by a sample stage 106. Further, the moving stage 124 can translate the sample stage two-dimensionally in accordance with a control signal from the control device 116, thereby changing the position where the electron beam 107 scans the sample 105.

  FIG. 2 is a flowchart showing a focus adjustment procedure. In FIG. 2, focus adjustment is performed in accordance with the procedures 201 to 209 with the above-described apparatus configuration. First, in 202, an image of each detector at each focal position is captured while the focal position is shifted by the objective lens control circuit 112. In this step, it is possible to obtain an image set in which the focus state gradually changes as the focal position moves.

  Next, in 203, the images are synthesized. The image acquired in 202 is calculated based on mathematical expressions registered in advance by the image processing means 119.

  FIG. 3 is an explanatory diagram for explaining the processing content of 203. At 202, an image is captured while changing the focal position for each detector. Reference numerals 301, 302, and 303 denote groups of images from detectors of secondary electrons, reflected electrons (L), and reflected electrons (R), respectively. Here, the reflected electron (L) indicates an electron detected on one side of the detector 123 of FIG. 1, and the reflected electron (R) indicates an electron detected on the other side of the detector 123. . The vertical direction represents the height direction, and the focus state gradually changes as the focal position moves. Using these images, the images (304, 305, 306) captured at the same focal position are calculated by the following Equation 1 to create a composite image 308.

なお、この式において、ＳＥ、Ｌ及びＲは、それぞれ２次電子像、反射電子像であるＬ像およびＲ像の加算要素を示し、α、β及びγは、それぞれＳＥ、Ｌ及びＲに対する係数を示す。

In this equation, SE, L, and R indicate the addition elements of the secondary and reflected electron images, L and R, respectively, and α, β, and γ are coefficients for SE, L, and R, respectively. Indicates.

  In this way, a composite image is calculated for each of the images 301 to 303, and an image group 307 is created. In the above formula 1, an image obtained by adding the difference between the reflected electron images to the secondary electron image at a specific ratio is output, and a secondary electron image with enhanced shadow contrast can be obtained. The above formula 1 is not limited to this content, and the addition elements and coefficients are not limited to this example.

  Next, in step 203, an evaluation value for evaluating the focus state is calculated. The evaluation value is calculated based on the differential value of the image. Pattern edges and the like are easy to understand, but in the focused state, the amount of change in shading is larger in the contour portion of the image than in the blurred state. From this, the sum of the pixel values after differentiating the image is calculated, and the value is used as the evaluation value. The place where the evaluation value is maximized is considered to be equal to the focal position. Specifically, a differential filter such as Laplacian is applied to the image data, and the sum of the resulting pixel values is calculated.

  About this evaluation value, when a graph with the evaluation value as shown in FIG. 5 as the vertical axis and the focal position (Z) as the horizontal axis is considered, it is approximated by a curve with the evaluation value at the in-focus position being the maximum. Is possible. In addition, a parabola curve, a polynomial curve, etc. are used for the curve at this time, ie, a fitting function. Therefore, the in-focus position can be found from the peak position by applying the fitting function to the evaluation value data.

  At this time, the deviation of the calculated fitting function from the original data is evaluated, and if the error between the calculated fitting function and the original data is not large, it can be determined that the data can withstand the calculation of the in-focus position. The control amount of the objective lens for obtaining the position and adjusting the focal point to the in-focus position can be calculated.

  If the error is large, there is a case where the correct peak position cannot be calculated because the deviation from the fitting function is large. For example, consider the case where one detector has sufficient S / N for the same location and the other detector does not have sufficient S / N. In the former case, the evaluation value graph is as shown in FIG. The horizontal axis of this graph is the focus position (Z), the vertical axis is the evaluation value, and the black circle 401 is a plot of the evaluation value from each image. When the S / N is sufficient, the fixed focus function 402 is followed, so that the just focus position can be correctly calculated by obtaining the peak position 403 of the fitting function. However, in the latter case where the detector is not properly selected and the S / N such as the pattern is not good, the noise component increases, and the error is larger than that of the black circle 401 when the evaluation value is normal like the white circle 501. Value. Even if fitting is performed in this state, the peak position 403 of the fitting function 402 indicating the correct just focus is not pointed, and an erroneous position such as the peak position 503 is easily determined as a peak.

  In this way, fitting is performed with a prescribed fitting function, and an error between the function and the evaluation value is a constant guideline as to whether the selected image is suitable for automatic focus adjustment. If the data is less than a certain error, the process moves to step 206, and the focal position is calculated using the same data. If it is not suitable, the process moves to step 208, the focus lens is not calculated and the state of the objective lens and the like is returned to the state before the start, the process moves to step 209, and the automatic focus adjustment process is terminated.

  After calculating the peak position in step 206, the process proceeds to step 207, where a signal is sent to the objective lens control circuit 112 so that the focus is adjusted to the in-focus position, and the focus is adjusted. Since the peak position of the evaluation value graph, the peak position 403 in FIG. 4 corresponds to the just focus position, a control amount corresponding to this position is transmitted. After the adjustment is completed, the process proceeds to step 209, and the automatic focus adjustment process is ended.

  FIG. 7 shows a flowchart of the second embodiment. In the second embodiment, in addition to the contents of the first embodiment, an example in which the user can set the calculation contents is shown. The difference from the first embodiment is that the user sets the calculation content immediately before the step 701 shown in FIG. 7, that is, immediately before the execution of the automatic focus adjustment, and calculates the composite image based on the set calculation content. The contents of this calculation can be designated by the user with the contents shown in the GUI as shown in FIG.

  A screen 601 shown in FIG. 6 is a GUI for setting a coefficient (that is, an image addition ratio) for each detector of Equation 1 described above. With this GUI, the mixing ratio of each image of Formula 1 can be set uniquely, and the focus accuracy can be increased according to the image. Here, an example in which the slide bars 605, 606, and 607 can be adjusted and set using the mouse 121 is shown, but a setting method such as inputting a numerical value directly using the input means of the keyboard 120 is shown in this screen example and means. It is not limited. In addition, the images to be mixed can be designated by the buttons 602, 603, and 604. In the case of this example, the setting for using the detector 1 and the detector 2 for calculation is shown, but the display method and the setting method are not limited to this example. In addition, instead of setting the mixing ratio of each image individually, it may be possible to select from a plurality of options. For example, when an area 610 is clicked, a screen 611 is displayed, and options 612, 613, and 614 corresponding to the purpose are set in advance so that the user can select items according to the purpose. When the option 612 is selected, a screen 615 is displayed. In actual calculation, a mixing ratio corresponding to the contents indicated by each option is set as a preset, and the value is used. In this case, since it is easy to select a target means for each application, it is easy for a user unfamiliar with the apparatus to set. The display and selection means of the screen 611 are not limited to this screen. The function of the detector automatic selection setting button 609 will be described in a third embodiment.

  When these settings are made, confirmation can be easily made by displaying an example of the image after calculation as shown in a screen 608. As the image displayed at this time, an image currently captured may be used, or an image stored by the user may be used. Regarding the method for saving the image, a button for saving the image may be prepared so that the user can save the image at an arbitrary timing. In addition, the imaging method is not limited to the above embodiment, such as preparing an image in advance or using an image saved in another operation. Further, the screen display is not essential. In addition, since the process after step 702 is the same as that after step 202 of the first embodiment, the details are omitted.

  FIG. 8 shows a flowchart of the third embodiment. This figure is basically the same as that of the second embodiment, but the image composition formula and the parameter selection method are different. This part is a function for automatically setting a detector for acquiring an image. FIG. 9 is a diagram for explaining parameters used in the calculation method selection process according to the third embodiment.

  When the automatic selection setting button 609 of the detector shown in FIG. 6 is selected, parameter sets 903, 904, and 905 as shown in the functional diagram 901 of FIG. Also, by providing step 810, mathematical formulas and parameters are determined before image calculation as shown in the functional diagram 902 of FIG. As the determination method, a mathematical expression and a parameter having the largest number of executions are set from the previously stored history data. As a result, an image effective for focus adjustment can be accurately synthesized, and the focus adjustment can be performed efficiently. Other processes in steps 801 to 809 are the same as those in steps 701 to 709 corresponding to the second embodiment, and detailed description thereof is omitted.

  If the error from the fitting function is large in step 805 and is not suitable for evaluation, the process moves to step 811 to execute evaluation using another calculation method. If there is an arithmetic method that has not yet been executed among the steps 903, 904, and 905 in step 811, the arithmetic method is reset in step 810 and the evaluation is executed. If all the calculation methods have been confirmed, the process moves to step 808, and the process ends in step 809 without executing the automatic focus adjustment. The method of resetting the calculation method in step 811 may be selected according to the order, but may be selected by setting a priority order from the following calculation method switching methods.

  The switching of the calculation method is not limited to the example given above, and if it is executed with a certain number of the same calculation method, processing based on probability and statistics, such as not making the determination based on the calculation method as a standard May be performed. In addition, the determination processing may be performed based on information on the wafer such as coordinates in the chip and the position of the chip, information on the process through which the wafer has passed, and an inspection apparatus. For example, for a wafer that has passed a specific process, if an image synthesized under the condition of parameter 903 is particularly effective in a specific part of the chip, auto-focusing can be efficiently performed by setting parameter 903 from the beginning in that coordinate range. Adjustments can be performed. Further, as data for these determination criteria, data collected during wafer observation may be used, data prepared in advance may be used, or they may be used in combination. The history information can also be set to a function that can be deleted according to the user's designation, such as the history deletion button 617 in FIG.

  When the user sets these processes, an automatic setting item can be provided and set as one of the settings in the GUI for setting the detector, like the automatic setting button 609 in FIG. However, this is only an example and the present invention is not limited to this. For example, an automatic setting item is provided in the whole automatic focus adjustment or other than the automatic focus adjustment setting as shown in the option 612 in FIG. Can be selected automatically. Although no setting items are displayed on the GUI, when none of the detectors in the buttons 602, 603, and 604 is selected, the detector may be automatically selected in the same manner.

  As described above, according to the embodiment of the present invention, when there are a mixture of images that can be picked up only by a specific detector, the purpose of performing stable automatic focus adjustment is to use an electron microscope having a plurality of detectors. Autofocus on a single image by capturing an image for each detector while changing the focus position (Z), creating a composite image using part or multiple images, and evaluating the image Even when adjustment was difficult, the just focus position could be detected.

The block diagram of the scanning electron microscope of Example 1, 2, 3 of this invention. The flowchart explaining the processing content of Example 1 of this invention. The figure explaining the image acquisition and calculation object in the autofocus function of Example 1 of the present invention. The normal profile graph which the evaluation value of Example 1 of this invention takes. The abnormal profile graph which the evaluation value of Example 1 of this invention takes. The figure which shows GUI which sets the calculation content of Example 2 of this invention. The flowchart explaining the processing content of Example 2 of this invention. The flowchart explaining the processing content of Example 3 of this invention. The figure explaining the parameter used by the calculation method selection process of Example 3 of this invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 101 ... Electron gun, 102 ... Condenser lens, 103 ... Deflector, 104 ... Objective lens, 105 ... Sample, 106 ... Sample stand, 107 ... Electron beam, 108 ... Secondary electron, 109 ... Reflected electron, 110 ... Lens control circuit DESCRIPTION OF SYMBOLS 111 ... Deflection control circuit, 112 ... Objective lens control circuit, 113 ... Analog / digital converter, 114 ... Address control circuit, 115 ... Image memory, 116 ... Control means, 117 ... Display, 118 ... Computer, 119 ... Image processing Means: 120 ... keyboard, 121 ... mouse, 122 ... secondary particle detector, 123 ... backscattered electron detector pair, 124 ... moving stage.

Claims (17)

An electron lens that focuses the electron beam emitted from the electron source onto the sample;
A detector for detecting a secondary signal generated from the sample by irradiation of the electron beam, a pair of first and second detectors provided at a predetermined interval; and a third detector; ,
The focus of the electron beam with respect to the sample is changed a plurality of times, and the first, second, and third second signals are detected from the secondary signals detected by the first, second, and third detectors at each focus. Storage means for generating and storing next signal data;
Data calculation means for performing a combination calculation process for combining the first, second, and third secondary signal data at the respective focal points;
An evaluation value is calculated based on an intensity signal obtained by performing predetermined data conversion processing on the secondary signal data at each focus subjected to the synthesis operation, and calculated based on the evaluation value corresponding to each focus. A scanning electron microscope comprising: a focus adjusting unit that adjusts a focus of the electron beam with respect to the sample by using a focus. In the description of claim 1,
In the scanning electron microscope, the combining calculation processing combines the first, second, and third secondary signal data at the focal point of the electron beam based on a preset calculation formula. . In the description of claim 2,
The calculation formula is the following formula (1), a scanning electron microscope.
Formula 1: αSE + βL + γR
(Here, α, β, and γ indicate coefficients, and L, R, and SE indicate the first, second, and third secondary signal data, respectively.) In the description of claim 3,
A scanning electron microscope characterized in that the coefficients α, β, and γ can be set to desired values using input means. In the description of claim 1,
The synthesis calculation processing is performed by adding a difference between the first secondary signal data corresponding to each focal point and the second secondary signal data at a predetermined ratio to the third secondary signal data at each focal point. A scanning electron microscope characterized by adding. In the description of claim 1,
The focus adjustment unit calculates a pixel value by performing a differentiation operation on the secondary signal data at each focus subjected to the synthesis calculation process, calculates an evaluation value based on a sum of the pixel values, A scanning electron microscope characterized in that the focus of the electron beam with respect to the sample is adjusted using a focus calculated based on a maximum value. In the description of claim 6,
The scanning electron microscope characterized in that the focus adjusting means controls the electron lens based on a focus calculated based on a maximum value of the evaluation value. In the description of claim 1,
The focus adjustment unit calculates a pixel value by performing a differentiation operation on the secondary signal data at each focus subjected to the synthesis calculation process, calculates an evaluation value based on a sum of the pixel values, A scanning electron microscope characterized in that a fitting function that matches a value is determined, and a focal point that gives a peak value of the fitting function is used to adjust a focal point of the electron beam with respect to the sample. In claim 8,
The focus adjusting unit calculates an error with respect to the fitting function, and when the error is within a predetermined reference error range, a focus that gives a peak value of the fitting function is used to focus the electron beam on the sample. A scanning electron microscope characterized by adjusting the angle. In the description of claim 9,
A scanning electron microscope characterized in that when the error is outside a predetermined reference error range, the focus of the electron beam with respect to the sample is maintained. An electron lens that focuses the electron beam emitted from the electron source onto the sample;
A detector for detecting a secondary signal generated from the sample by irradiation of the electron beam, a pair of first and second detectors provided at a predetermined interval; and a third detector; ,
The focus of the electron beam with respect to the sample is changed a plurality of times, and the first, second, and third second signals are detected from the secondary signals detected by the first, second, and third detectors at each focus. Storage means for generating and storing next signal data;
Data computing means for computing the first, second, and third secondary signal data at each focus;
Input means for inputting data necessary for calculation processing in the data calculation means from the outside;
Image display means for displaying a command menu for indicating the result of the arithmetic processing executed by the data arithmetic means, the input data from the input means and the content of the arithmetic processing executed by the data arithmetic means;
An evaluation value is calculated based on an intensity signal obtained by performing a predetermined data conversion process on the secondary signal data at each of the focal points subjected to the arithmetic processing performed by the data arithmetic means, and the corresponding to each focal point A scanning electron microscope comprising: a focus adjusting unit that adjusts a focus of the electron beam with respect to the sample using a focus calculated based on an evaluation value. In the description of claim 11,
The calculation processing executed by the data calculation means is a combination of the first, second, and third secondary signal data at the focus of the electron beam based on a preset calculation formula. A scanning electron microscope characterized by being. In the description of claim 12,
Data for changing a coefficient set in advance in the calculation formula is input by the input unit. In the description of claim 13,
A scanning electron microscope comprising storage means for storing a coefficient of the calculation formula, and when the coefficient is changed, the changed coefficient is added to the storage means. In the description of claim 14,
The scanning electron microscope according to claim 1, wherein a value that is frequently used for adjusting the focal point of the electron beam is selected from the coefficients stored in the storage unit. In the description of claim 15,
The scanning electron microscope characterized in that the coefficient or the number of times stored in the storage means is erased by the input means.   A scanning electron microscope comprising a plurality of detectors for detecting secondary signals generated from a sample by irradiating the sample with an electron beam, and arithmetic means for combining the signals obtained by the detector, At least two of the detectors are arranged symmetrically with respect to the electron beam, and the focus of the electron beam is adjusted based on a signal of each detector or a synthesized signal. Type electron microscope.

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