Abstract:
In imaging a sample using an electron microscope, in order to reduce a time for focusing, a scanning range of a Z coordinate is reduced to complete focusing by obtaining SEM images such that: for a first predetermined number of portions, focal positions of an electron beam in obtaining each SEM image are moved in a predetermined range; then, a curved surface shape of the surface of the sample is estimated by using information relating to the focal positions of the electron beam in the first predetermined number of portions; after the images are taken, the range in which the focal positions of the electron beam are moved for scanning the electron beam on the surface of the sample is made to be narrower than the predetermined range by using the curved surface information estimated, thereby performing scanning to take the images of the sample.

Description:
BACKGROUND 
       [0001]    The present invention relates to a defect inspection method and a defect inspection device used to inspect the quality of devices such as a semiconductor product, a magnetic head, a magnetic disk, a solar battery, an optical module, a light emitting diode, or a liquid crystal display panel, each of which is formed by repeating film formation, resist coating, exposure, development, etching, and the like on a substrate. Such a defect inspection method and a defect inspection device, for example, are used to inspect the dimension of a formed pattern, and the size and shape of a defect that has occurred, and the like. The present invention particularly relates to a defect inspection method and a defect inspection device which use a digital camera for an electron microscope or an optical microscope. 
         [0002]    In recent years, electron microscopes are widely used to inspect the qualities of semiconductor products, magnetic heads, magnetic disks, solar batteries, optical modules, light emitting diodes, liquid crystal display panels, and the like, for example, to inspect a dimension of a formed pattern and the size and shape of a defect occurred. 
         [0003]    In order to observe a pattern or a defect on a substrate using an electron microscope, a stage that holds thereon a substrate to be observed is moved so that specified XY coordinates are located near the center of an image. Next, a Z coordinate of the stage is moved so that the pattern or the defect is focused, and the image is acquired. 
         [0004]    JP-A-2009-194272 (Patent Document 1) describes a method for registering an offset, called a focus map, of a Z coordinate in order to shorten a scanning range of the Z coordinate (in the direction of a normal to a stage surface on which a substrate is placed) and performing focusing the offset as a standard. In this method, a Z coordinate that causes a focal point to be adjusted to each of various positions on a certain substrate is measured, and a curved surface is approximated for measured XYZ coordinate groups, and the approximated curved surface is registered in an electron microscope as a focus map. Next, when another substrate is observed using the electron microscope, a Z coordinate that causes the focal point to be adjusted to certain XY coordinates is assumed to be close to the Z coordinate corresponding to the XY coordinates of the registered focus map, the Z coordinate is moved around the assumed Z coordinate, and focusing is performed. 
       SUMMARY 
       [0005]    In order to observe a pattern or a defect on a substrate using an electron microscope, it is necessary to slowly move a Z coordinate in order to perform focusing while a depth of field of the electron microscope is shallow. Taking a long time to perform focusing is a problem to be solved. 
         [0006]    In the method described in Patent Document 1, when an assumption that Z coordinates corresponding to the XY coordinates are close to each other between substrates is not sufficiently satisfied, there is a problem that focusing is delayed. 
         [0007]    The present invention provides a defect inspection method and a defect inspection device, which can reduce a scanning range in which a Z coordinate is moved in order to perform focusing, thereby reducing the time to perform the focusing as a result of the shortening, even if Z coordinates corresponding to XY coordinates are not close to each other between substrates. 
         [0008]    In the method described in Patent Document 1, a cumbersome task of measuring Z coordinates corresponding to many XY coordinates for an arbitrary substrate and registering a focus map is required. The present invention, however, provides a defect inspection method and a defect inspection device, which do not need such a cumbersome task. 
         [0009]    In order to solve the aforementioned problems, according to the present invention, a defect inspection device includes: a scanning electron microscope that irradiates a sample with a focused electron beam, performs scanning, and thereby acquires an SEM image of the sample; image processing unit which processes the SEM image acquired by imaging the sample with the scanning electron microscope; output unit which outputs, on a screen, a result of processing the SEM image of the sample with the image processing unit; and control unit which controls the scanning electron microscope, the image processing unit, and the output unit. In the defect inspection device, when the control unit controls the scanning electron microscope for sequentially imaging a plurality of portions on the sample, the control unit causes the scanning electron microscope: in a first predetermined number of portions, to perform the scanning by moving a focal point of the electron beam in a normal direction relative to a surface of the sample in a predetermined range; to adjust the focal point of the electron beam to the surface of the sample; and to image the sample; then, to estimate a curved surface shape of the surface of the sample by using information about positions to which the focal point of the electron beam has been adjusted on the surface of the sample in the first predetermined number of portions; after imaging the sample in the first predetermined number of portions, to use information about the estimated curved surface to perform the scanning by moving the focal point of the electron beam in the normal direction in a narrower range than the predetermined range for the scanning that has been performed in the first predetermined number of portions in order to adjust the focal point of the electron beam to the surface of the sample; to adjust the focal point of the electron beam to the surface of the sample; and to image the sample. 
         [0010]    In addition, in order to solve the aforementioned problems, according to the present invention, a defect inspection method comprising the steps of: sequentially acquiring SEM images of a plurality of portions on a sample by irradiating the plurality of portions on a sample with an electron beam focused by a scanning electron microscope to perform scanning; processing the SEM images obtained by sequentially imaging the plurality of portions on the sample with the scanning electron microscope and inspecting the sample; and outputting a result of processing the SEM images of the sample. In the defect inspection method, the sequential acquisition of the SEM images of the plurality of portions on the sample with the scanning electron microscope includes the steps of: performing the scanning for a first predetermined number of portions by moving a focal point of the electron beam in a normal direction relative to a surface of the sample in a predetermined range, adjusting the focal point of the electron beam to the surface of the sample, and imaging the sample; estimating a curved surface shape of the surface of the sample by using information about positions to which the focal point of the electron beam has been adjusted on the surface of the sample in the first predetermined number of portions; and after imaging the sample in the first predetermined number of portions, using information about the estimated curved surface to perform the scanning by moving the focal point of the electron beam in the normal direction in a narrower range than the predetermined range for the scanning that has been performed in the first predetermined number of portions in order to adjust the focal point of the electron beam to the surface of the sample, adjusting the focal point of the electron beam to the surface of the sample, and imaging the sample. 
         [0011]    Furthermore, in order to solve the aforementioned problems, according to the present invention, a defect inspection method comprising the steps of: sequentially acquiring SEM images of a plurality of portions on a sample by sequentially performing, on a plurality of portions of the surface of the sample, scanning with a focal point of an electron beam focused by a scanning electron microscope, the focal point adjusted to a surface of a sample, and imaging the sample; processing the sequentially acquired SEM images of the plurality of portions on the sample and inspecting the sample; and outputting results of the inspection. In the defect inspection method, the adjusting of the focal point of the electron beam focused by the scanning electron microscope on the surface of the sample includes the steps of: performing the scanning in a first predetermined number of portions by moving the focal point of the electron beam in a normal direction relative to the surface of the sample in a predetermined range, adjusting the focal point of the electron beam to the surface of the sample, and imaging the sample; after imaging the sample in the first predetermined number of portions, performing the scanning by moving the focal point of the electron beam in a narrower range than the predetermined range in which scanning has been performed in the first predetermined number of portions using information indicating that the focal point of the electron beam has been adjusted to the surface of the sample for each of the first predetermined number of portions, adjusting the focal point of the electron beam on the surface of the sample, and imaging the sample. 
         [0012]    According to the present invention, the defect inspection methods and the defect inspection device can reduce the scanning range of the Z coordinate to be moved in order to perform focusing for various substrates and can thereby reduce the time to complete the focusing. In addition, Z coordinates corresponding to many XY coordinates are measured for an arbitrary substrate in advance, and an image can be acquired without a cumbersome task of registering a focus map. 
         [0013]    These features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a block diagram illustrating an example of an outline of an electron microscope device. 
           [0015]      FIG. 2  is a flow diagram illustrating an example of a flowchart of an image acquisition program. 
           [0016]      FIG. 3  is a diagram illustrating an example of image acquisition coordinate data. 
           [0017]      FIG. 4  is a plan view of a wafer, illustrating an example of a map including image acquisition coordinate data. 
           [0018]      FIG. 5  is a flow diagram illustrating an example of a flowchart of an image acquisition order determination program. 
           [0019]      FIG. 6  is a plan view of the wafer, illustrating an example of standard coordinates. 
           [0020]      FIG. 7  is a plan view of the wafer, illustrating initial image acquisition coordinate groups. 
           [0021]      FIG. 8  is a diagram illustrating an example of image acquisition coordinate data after alignment. 
           [0022]      FIG. 9  is a diagram illustrating an example of image acquisition coordinate data after images of the initial image acquisition coordinate groups are acquired. 
           [0023]      FIG. 10  is a diagram illustrating an example of image acquisition coordinate data after a curved surface is approximated. 
           [0024]      FIG. 11  is a graph illustrating an example of an outline diagram with respect to a time axis. 
           [0025]      FIG. 12  is a flow diagram illustrating an example of a flowchart of the image acquisition program. 
           [0026]      FIG. 13  is a graph illustrating an example of an outline diagram with respect to a time axis. 
           [0027]      FIG. 14  is a front view of a screen showing an example of a graphical user interface displaying an approximated curved surface. 
           [0028]      FIG. 15  is a front view of a screen showing an example of a graphical user interface displaying differences between the curved surface and actual measured values. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0029]    Hereinafter, embodiments of the present invention are described in detail with reference to the accompanying drawings. 
       First Embodiment 
       [0030]      FIG. 1  is a diagram illustrating an example of an outline of a scanning electron microscope (SEM) as an example of a defect inspection method and defect inspection device according to the first embodiment. The scanning electron microscope has an electron source  201  for generating a primary electron  208 , an acceleration electrode  202  for accelerating the primary electron, a focusing lens  203  for focusing the primary electron, an aperture  212  for narrowing a beam, a deflector  204  for two-dimensionally scanning and deflecting the primary electron, a blanking electrode  221  for temporarily stopping irradiation of a substrate  206  with the beam, and an objective lens  205  for focusing the primary electron on the substrate  206  such as a wafer. Reference numeral  207  indicates a driving stage that holds the substrate  206  thereon. Reference numeral  210  indicates a detector that detects a secondary electron  209  generated from the substrate  206 . Reference symbols  220   a  and  220   b  indicate reflected electron detectors that detect a reflected electron  219 . In  FIG. 1 , the reflected electron detectors  220   a  and  220   b  face to each other and detect different components of the reflected electron  219 . Reference numeral  211  indicates a digital converter (A/D converter) that digitalizes the detected signals. These parts are connected to an overall controller  213  through a bus  218 . 
         [0031]    The scanning electron microscope according to the present embodiment further includes a central processing unit (CPU)  214 , a primary storage device  215 , a secondary storage device  216 , and an input and output unit  217  that is a keyboard, a mouse, a display, a printer, a network interface, or the like. In addition, image acquisition coordinate data  230 , an image acquisition program  231 , an image acquisition order determination program  232 , a curved surface approximation program  233 , an image processing program  234 , and the like are stored in the secondary storage device  216 . In the image acquisition coordinate data  230 , coordinate groups from which images are to be acquired in a surface of the substrate  206  are described. The image acquisition program  231  is used to cause the driving stage to move and acquire an image. The image acquisition order determination program  232  is used to align the image acquisition coordinate data and determine the order of acquiring images. The curved surface approximation program  233  is used to approximate a curved surface such as a B-spline surface or a response surface from XY coordinates from which the images are acquired and Z coordinates that cause a focal point to be adjusted to the XY coordinates. The image processing program  234  is used to process the acquired images (SEM images), extract a defect, calculate an image characteristic amount of the defect, classify the defect, and calculate dimensions of patterns included in the acquired processed images and a distance between the patterns. These programs are read from the secondary storage device  216  into the primary storage device  215  and run by the central processing unit (CPU)  214 . 
         [0032]      FIG. 2  illustrates an example of a flowchart of the image acquisition program  231 . According to the image acquisition program  231 , the image acquisition coordinate data  230  is read in step S 301 , and the order of acquiring images of the coordinate groups described in the read image acquisition coordinate data  230  is determined in step S 302 . The image acquisition coordinate data  230  read in step S 301  may be design data (CAD data) about a circuit pattern formed on the substrate  206  or positional information about a defect (pattern defect, foreign material defect, or the like) inspected and detected by another inspection device. 
         [0033]    In this flowchart, the order of acquiring the images of the coordinate groups is defined as the order of coordinates (X1, Y1), (X2, Y2), . . . , and (XN, YN). In addition, a Z coordinate that causes the focal point to be adjusted to the coordinates (XN, YN) in order to acquire an image of the coordinates (XN, YN) is defined as ZN, and the three-dimensional coordinates are defined as (XN, YN, ZN). Next, a variable N is substituted with zero in step S 303 , and the variable N is incremented by 1 in step S 304 . If the variable N exceeds the number of the coordinate groups described in the image acquisition coordinate data  230  in conditional branching step S 305 , the program is terminated. 
         [0034]    On the other hand, if the variable N does not exceed the number of the coordinate groups described in the image acquisition coordinate data  230  in conditional branching step S 305 , the process proceeds to step S 306 . In step S 306 , the variable N is compared with the number of the images to be initially acquired, which is specified in step S 302 , and the process proceeds to any of steps S 307 , S 308 , and S 310 . If the variable N is smaller than a value obtained by adding 1 to the number of the images to be initially acquired, the process proceeds to step S 310 . If the variable N is equal to the value obtained by adding 1 to the number of the images to be initially acquired, the process proceeds to step S 307 . If the variable N is larger than the value obtained by adding 1 to the number of the images to be initially acquired, the process proceeds to step S 308 . 
         [0035]    Step S 310  is performed when the variable N is in a range of 1 to the number of the images to be initially acquired. In step S 310 , the stage is moved so that an image of the coordinates (XN, YN) can be acquired, while performing the scanning in a range of (a base point−β) to (the base point+β) of Z coordinate, and the focusing is completed. The base point means an estimated value of an average Z coordinate of the surface of the substrate, and β is a fixed value that is larger than a described later. Step S 307  is performed when the variable N is equal to the value obtained by adding 1 to the number of the images to be initially acquired. In step S 307 , a B-spline surface is approximated for a number N of three-dimensional coordinates from (X1, Y1, Z1) to (XN−1, YN−1, ZN−1), and the approximated curved surface is treated as a focus map. 
         [0036]    In step S 308 , a value ZN′ for coordinates (XN, YN) of the approximated curved surface is calculated. The value ZN′ is an estimated value of a Z coordinate that is estimated to cause the focal point to be adjusted to the coordinates (XN, YN) in order to acquire an image of the coordinates (XN, YN). 
         [0037]    In step S 309 , the stage is moved so as to enable an image of the coordinates (XN, YN) to be acquired, while performing the scanning in a scanning range of (ZN′−α) to (ZN′+α) of Z coordinate, and the focusing is completed. The Z coordinate that causes the focal point to be adjusted to the coordinates (XN, YN) is regarded as ZN. The value a can be set to a smaller value than the value β by generating the focus map and calculating ZN′. As a result, the scanning range of the Z coordinate can be reduced, and the time to perform the focusing can be reduced. In step S 311 , an image of the coordinates (XN, YN) to which the focal point is adjusted is acquired. In step S 312 , the acquired image is processed according to the image processing program  234  so that image processing of detecting a defect from the acquired image of the coordinates (XN, YN), extracting a characteristic amount of the defect and classifying the defect, or image processing of calculating a size of the defect or a pattern, or calculating a distance between patterns from a characteristic amount of the image is performed. In step S 313 , the three-dimensional coordinates (XN, YN, ZN) are registered in the secondary storage device  216  in order to generate or update the focus map in step S 307 . Then, the process returns to step S 304 . 
         [0038]      FIG. 14  illustrates an example of a graphical user interface for displaying the curved surface approximated in step S 307 . A graphical user interface  800  has a part  820  for displaying the approximate curved surface or the focus map, a part  810  for displaying the number of the wafer to be inspected, a part  840  for selecting the type of the focus map to be displayed, a part  841  for selecting a defect to be displayed simultaneously with the focus map, and a part  842  for displaying a scale for the display of the focus map. In the part  820  for displaying the focus map, an outer frame  821  of the wafer is displayed, and a notch  822  is displayed in order to recognize the orientation of the wafer. Since the focus map is calculated not for an overall surface of the wafer but for only a region on which products such as an integrated circuit are formed, the region is displayed as  823 . 
         [0039]    Colors are added to the region  823  in order to recognize bias of the focus map, and contour lines are depicted in the region  823 . The definition of the color addition is displayed as the scale  842 . In order to display the focus map, one of “Front”, “All”, and “Difference” is selected in the selection part  840 . If “Front” is selected, the curved surface approximated in step S 307  is displayed as the focus map on the basis of initial image acquisition coordinate groups. If “All” is selected, a B-spline surface is approximated from all input coordinate groups in the same manner as step S 307 , and the result of the approximation is displayed. 
         [0040]    If “Difference” is selected, a result obtained by calculating, for each of coordinate groups, a difference between the curved surface displayed when “Front” is selected and the curved surface displayed when “All” is selected is displayed. On this graphical user interface, the focus map and coordinates from which images are acquired can be simultaneously displayed. Whether or not simultaneously displaying the coordinates from which the images are acquired is selected using the selection part  841 . Regarding the selection method, there are four cases: the case where both “Front” and “Others” are not selected, the case where only “Front” is selected, the case where only “others” is selected, and the case where both “Front” and “Others” are selected. If “Front” is selected, initial image acquisition coordinate groups that are indicated by star signs  831  are displayed with overlapped with the focus map. If “Others” is selected, all other coordinates excluding initial image acquisition coordinate groups indicated by triangles  832  are displayed. 
         [0041]      FIG. 15  illustrates an example of a graphical user interface that displays differences between Z coordinates when the curved surface is approximated in step S 307  and Z coordinates when focusing is performed for acquisition of images. 
         [0042]    A graphical user interface  900  has a graph display part  920 , a part  910  for displaying the number of the wafer to be inspected, a part  931  for selecting data to be displayed, a part  932  for displaying, as a root mean square error (RMSE) represented by Equation 1, differences between Z coordinates of the approximated curved surface or focus map and the Z coordinates when the focusing is actually performed for the acquisition of the images, a part  933  for displaying and entering a value of a scanning range a for the focusing to be performed in step S 309 , and a part  934  for displaying and entering a result of estimating a probability of a focusing error in which the Z coordinate at which the focusing is completed is not in the scanning range of (ZN′−α) to (ZN&#39;+α) in step S 309 . 
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         [0043]    In the graph display part  920 , the abscissa indicates a Z coordinate when the focusing is completed for acquisition of an image, and the ordinate indicates a value estimated from the focus map. The graph display part  920  displays a distribution diagram in which a point is added for each group of coordinates from which images are acquired. A straight line  921  is a straight line on which values of the ordinate and abscissa are the same. It is apparent that the closer to the straight line  921 , the higher the accuracy of the focus map. A result that quantitatively indicates the accuracy is a value displayed in the RMSE display part  932 . Two broken lines  922  that are drawn in the distribution diagram are straight lines that indicate +α and −α of the scanning range of the Z coordinate. The drawing of the broken lines  922  coordinates with a value entered in the part  933  for displaying and entering a value of α. 
         [0044]    The original data from which the points are provided to the distribution diagram is determined by selecting any one of “with Front” and “without Front” in the data selection part  931 . If “with Front” is selected, Z coordinates that correspond to all XY coordinates from which images are acquired are targets. The initial image acquisition coordinate groups for generation of the focus map are included in “with Front”. If “without Front” is selected, Z coordinates that correspond to XY coordinates excluding the initial image acquisition coordinate groups from all coordinate groups from which images are acquired are targets. 
         [0045]    A value of the part  933  for displaying and entering the value of α and a value of the part  934  for displaying and entering the probability of the focusing error coordinate with each other. For example, when the RMSE is 22.7 nanometers as illustrated in the example of the distribution diagram, if a value that is 3.99 times as large as 22.7 or 90.6 is entered, the probability of the error or the probability that a point is provided in the outside of the broken lines  922  is 100 ppm. This calculation is equivalent with that, on an assumption that the RMSE is equal to a standard deviation, α is 3.99 times the standard deviation. Specifically, a probability of being larger by +α or smaller by −α than a probability density function, indicated by Equation 2, of a normal distribution is a result calculated according to Equation 3. In Equation 2, σ is the standard deviation or the RMSE in this case, and μ is the average of differences between ZN′ and ZN. 
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         [0046]      FIG. 3  illustrates an example of the image acquisition coordinate data  230  read in S 301 . The image acquisition coordinate data  230  includes a plurality of XY coordinates, and serial numbers are added to the XY coordinates. Points  2301  indicated in the leftmost column are the serial numbers of the coordinates, X  2302  indicated in the second column indicates the X coordinates, and Y  2303  indicated in the third column indicates the Y coordinates. In this example, 122 coordinates are illustrated. The present invention, however, is not limited to the 122 coordinates. The number of coordinates is arbitrary as long as the number of the coordinates is two or more. 
         [0047]      FIG. 4  illustrates an example in which the image acquisition coordinate data  230  illustrated in  FIG. 3  is visually illustrated. A circular frame  320  indicates the substrate that has a notch  321  that is a standard of a coordinate system. In this example, when the notch  321  is located on the lower side, a lowermost end on which a semiconductor product, a magnetic head, or the like is formed is defined as an X axis, and a leftmost end is defined as a Y axis. The image acquisition coordinate data  230  is read onto the XY coordinate system, and black triangles indicate points of the coordinate groups. For example, a triangle  331  indicates coordinates (81.4, 46.2) of the serial number (point)  2301  (illustrated in  FIG. 3 ) of 1, and a triangle  332  indicates coordinates (107.3, 46.2) of the serial number (point)  2301  (illustrated in  FIG. 3 ) of 2. 
         [0048]      FIG. 5  illustrates an example of a flowchart for step S 302  of the image acquisition program described with reference to  FIG. 2  or for the image acquisition order determination program  232  for determining the order of acquiring images. In step S 401 , standard coordinates that include a plurality of XY coordinates are read from the secondary storage device  216 . The standard coordinates are coordinate groups for which Z coordinates are recommended to be measured in order to generate the focus map. In step S 402 , image acquisition coordinates that are closest to each of the standard coordinate groups read in step S 401  are selected from the image acquisition coordinate data  230 . In step S 403 , the image acquisition coordinate data is aligned so that images of the selected plurality of image acquisition coordinates are initially acquired, and the order of acquiring the images is treated as an image acquisition order. 
         [0049]      FIG. 6  visually illustrates an example of the standard coordinates read in step S 401  of the image acquisition order determination program  232 . In this example, 10 points  410  are at the standard coordinates. The standard coordinates are preferably located near an edge portion of the wafer and distributed on the overall surface of the wafer. The standard coordinates may be registered in the secondary storage device  216  in advance, or set using the input and output unit  217  every time the substrate is observed. In this example, the 10 coordinates are illustrated as the standard coordinates. The standard coordinates, however, are not limited to the 10 coordinates. The number of the standard coordinates may be arbitrary as long as the number of the standard coordinates is two or more. 
         [0050]      FIG. 7  visually illustrates an example of the image acquisition coordinates selected in step S 402  of the image acquisition order determination program  232 . In this example, image acquisition coordinates positioned closest to the 10 points illustrated in  FIG. 6  are selected from among the coordinate groups indicated by the triangles in  FIG. 4 . Ten points indicated by star signs  420  are the selected image acquisition coordinates. 
         [0051]      FIG. 8  illustrates an example of the image acquisition order determined by the image acquisition order determination program  232 . In this example, the 10 coordinates that are indicated by the star signs  420  in  FIG. 7  and included in the image acquisition coordinate data  2301 ,  2302 , and  2303  illustrated in  FIG. 3  are described in a top portion. The coordinate groups indicated by the star signs  420  and described in the top portion are referred to as initial image acquisition coordinate groups  810 . 
         [0052]      FIG. 9  illustrates an example of the three-dimensional coordinates registered in step S 312  of the image acquisition program  231  described with reference to  FIG. 2 . This example indicates results of completely acquiring images of all the points of the initial acquisition coordinate groups  810  illustrated in  FIG. 8 , or results of registering coordinates from (X1, Y1, Z1) to (X10, Y10, Z10). 10 Z coordinates from which images are completely acquired, or Z coordinates  901  at which the focusing is completed are added to image acquisition coordinate data  801 ,  802 , and  803  aligned in the image acquisition order illustrated in  FIG. 8 . 
         [0053]      FIG. 10  illustrates an example of results of approximating the curved surface by the curved surface approximation program  233  or results of step S 307  of the image acquisition program  231  described with reference to  FIG. 2 . In  FIG. 10 , the results of approximating the B-spline surface relative to the results of registering the coordinates (X1, Y1, Z1) to (X10, Y10, Z10) are illustrated. The B-spline surface approximation is a method for generating a smooth, adjustable surface using positional vectors of 16 control points arranged in order and is widely used for a computer aided design (CAD) tool, as described in “S. Lee, G. Wolberg, S. Y. Shin: “Scattered Data Interpolation with Multilevel B-Splines”, IEEE Transactions on Visualization and Computer Graphics, Vol. 3, No. 3 pages 228-244 (1997):” (Non-Patent Document 1). Z coordinates corresponding to XY coordinates on the approximated B-spline surface are indicated by Z′  1001  on the rightmost column of the table. In this example, the B-spline surface approximation is used. The present invention, however, is not limited to the B-spline surface. A response surface or a Bezier curved surface may be approximated. 
         [0054]      FIG. 11  is an example of a schematic diagram illustrating the state in which the scanning range of the Z coordinate that is moved for the focusing is changed by executing the image acquisition program described with referenced to  FIG. 2 . In this schematic diagram, the image acquisition coordinates are reduced to 9 coordinates for convenience of the sheet. The abscissa indicates a time axis, and the ordinate indicates a Z coordinate that is moved for the focusing. A broken line  501  indicates a Z coordinate of the surface of the substrate. Arrows from  511  to  519  that extend in a vertical direction indicate Z-directional ranges of the Z coordinate to be moved in order to adjust the focal point to the image acquisition coordinates. 
         [0055]    After the image acquisition program is executed and the image acquisition order is determined, the Z coordinate is moved in the range indicated by the arrow  511  for coordinates from which an image is acquired first, and the focusing is performed. The center of the arrow  511  is a vertical position of the base point. The Z coordinate at which the focusing is completed is indicated by a star sign  521 . Next, the Z coordinate is moved in the range indicated by the arrow  512  for coordinates from which an image is acquired second, and the focusing is performed. The center of the arrow  512  is a vertical position of the base point. The Z coordinate at which the focusing is completed is indicated by a star sign  522 . Next, the Z coordinate is moved in the range indicated by the arrow  513  for coordinates from which an image is acquired third, and the focusing is performed. The center of the arrow  513  is a vertical position of the base point. The Z coordinate at which the focusing is completed is indicated by a star sign  523 . Next, the Z coordinate is moved in the range indicated by the arrow  514  for coordinates from which an image is acquired fourth, and the focusing is performed. The center of the arrow  514  is a vertical position of the base point. The Z coordinate at which the focusing is completed is indicated by a star sign  524 . 
         [0056]    In the schematic diagram, the first to fourth image acquisition coordinates are initial coordinate groups. Next, a curved surface is approximated for the first to fourth image acquisition coordinates and the Z coordinates indicated by the star signs  521 ,  522 ,  523 , and  524 . Z coordinates of a curved surface approximated for the fifth to ninth image acquisition coordinates, or estimated values of Z coordinates at which the focusing is estimated to be completed for the fifth to ninth image acquisition coordinates, are indicated by an alternate long and short dashed line  530 . 
         [0057]    Next, a range of an arrow  515  is determined for the fifth image acquisition coordinates so that the alternate long and short dashed line  530  is located at the center of the arrow  515 , the Z coordinate is moved, and the focusing is performed. The alternate long and short dashed line  530  crosses at the center of the arrow  515 . A Z coordinate at which the focusing is completed is indicated by a triangle  525 . Next, a range of an arrow  516  is determined for the sixth image acquisition coordinates so that the alternate long and short dashed line  530  is located at the center of the arrow  516 , the Z coordinate is moved, and the focusing is performed. A Z coordinate at which the focusing is completed is indicated by a triangle  526 . Next, a range of an arrow  517  is determined for the seventh image acquisition coordinates so that the alternate long and short dashed line  530  is located at the center of the arrow  517 , the Z coordinate is moved, and the focusing is performed. A Z coordinate at which the focusing is completed is indicated by a triangle  527 . Next, a range of an arrow  518  is determined for the eighth image acquisition coordinates so that the alternate long and short dashed line  530  is located at the center of the arrow  518 , the Z coordinate is moved, and the focusing is performed. A Z coordinate at which the focusing is completed is indicated by a triangle  528 . Next, a range of an arrow  519  is determined for the ninth image acquisition coordinates so that the alternate long and short dashed line  530  is located at the center of the arrow  519 , the Z coordinate is moved, and the focusing is performed. A Z coordinate at which the focusing is completed is indicated by a triangle  529 . 
         [0058]    According to the present embodiment, a curved surface is approximated on the basis of focal point positional information about initial image acquisition coordinate groups, a Z-directional scanning range of which the center is on the curved surface is reduced, and whereby the focusing can be completed in a shorter time. 
       Second Embodiment 
       [0059]    The first embodiment describes the example in which a curved surface is approximated only once when the acquisition of the images of the initial image acquisition coordinate groups are completed and the focus map is generated. The second embodiment describes an example in which the focus map is updated at any time. 
         [0060]      FIG. 12  is a flowchart of the image acquisition program  231  and illustrates a different example from  FIG. 2 . According to the image acquisition program, the image acquisition coordinate data  230  is read in step S 1201 , and the order of acquiring images of the coordinate groups described in the read image acquisition coordinate data  230  is determined in step S 1202 . Next, the variable N is substituted with zero in step S 1203 , and the variable N is incremented by 1 in step S 1204 . If the variable N exceeds the number of the coordinate groups described in the image acquisition coordinate data  230  in conditional branching step S 1205 , the program is terminated. 
         [0061]    On the other hand, if the variable N does not exceed the number of the coordinate groups described in the image acquisition coordinate data  230  in conditional branching step S 1205 , the process proceeds to step S 1206 . In step S 1206 , the variable N is compared with the number of the images to be initially acquired, specified in step S 1202 , and the process proceeds to any of steps S 1207  and S 1210 . If the variable N is smaller than a value obtained by adding 1 to the number of the images to be initially acquired, the process proceeds to step S 1210 . If the variable N is not smaller than the value, the process proceeds to step S 1207 . 
         [0062]    Step S 1210  is performed when the variable N is in the range of 1 to the number of the images to be initially acquired. In step S 1210 , the stage is moved so that an image of the coordinates (XN, YN) can be acquired and the Z coordinate is moved in a scanning range of (a base point−β) to (the base point+β), and the focusing is performed. 
         [0063]    The base point means an estimated value of an average Z coordinate of the surface of the substrate, and β is a fixed value that is larger than a described later. Step S 1207  is performed when the variable N is equal to the value obtained by adding 1 to the number of the images to be initially acquired. In step S 1207 , a B-spline surface is approximated for a number N of three-dimensional coordinates from (X1, Y1, Z1) to (XN−1, YN−1, ZN−1), and the approximated curved surface is treated as a focus map. 
         [0064]    In step S 1208 , a value ZN′ corresponding to coordinates (XN, YN) of the approximated curved surface is calculated. The value ZN′ is an estimated value of a Z coordinate that is estimated to adjust the focal point in order to acquire an image of the coordinates (XN, YN). In step S 1209 , the stage is moved so that an image of the coordinates (XN, YN) can be acquired and the Z coordinate is moved in a scanning range of (ZN′−γ) to (ZN′+γ), and the focusing is performed. The Z coordinate that causes the focal point to be adjusted to the coordinates (XN, YN) is regarded as ZN. The value γ can be set to a smaller value than the value β by generating a focus map and calculating ZN′. As a result, the scanning range of the Z coordinate can be reduced, and the time to complete the focusing can be reduced. In addition, the process using γ takes a shorter time than the process using a in step S 309  of  FIG. 2 . Specifically, as the number of images to be acquired is increased, the value γ can be reduced. 
         [0065]    In step S 1211 , an image of the coordinates (XN, YN) to which the focal point is adjusted is acquired. In step S 1212 , the acquired image is processed according to the image processing program  234 , and then image processing as follows is performed. One is an image processing which includes: detecting a defect from the acquired image of the coordinates (XN, YN), extracting a characteristic amount of the defect, and classifying the defect. Other one is an image processing which includes: calculating a size of the defector a pattern, or calculating a distance between patterns from a characteristic amount of the image. In step S 1213 , the three dimensional coordinates (XN, YN, ZN) are registered in the secondary storage device  216  in order to generate or update the focus map in step S 1207 . Then, the process returns to step S 1204 . 
         [0066]      FIG. 13  is a schematic diagram illustrating an example of the state in which the scanning range of the Z coordinate to be moved for the focusing is changed by executing the image acquisition program described in  FIG. 12 . In this schematic diagram, the image acquisition coordinates are reduced to 9 coordinates for convenience of the sheet. The abscissa indicates a time axis, and the ordinate indicates the Z coordinate that is moved for the focusing. A broken line  501  indicates a Z coordinate of the surface of the substrate. Arrows from  511  to  515  and  546  to  549  that extend in the vertical direction indicate scanning ranges in Z direction for adjusting the focal point to each of the image acquisition coordinates. 
         [0067]    After the image acquisition program  231  is executed and the image acquisition order is determined, the Z coordinate is moved in the range indicated by the arrow  511  for coordinates from which an image is acquired first, and the focusing is performed. The Z coordinate at which the focusing is completed is indicated by a star sign  521 . Next, the Z coordinate is moved in the range indicated by the arrow  512  for coordinates from which an image is acquired second, and the focusing is performed. The Z coordinate at which the focusing is completed is indicated by a star sign  522 . Next, the Z coordinate is moved in the range indicated by the arrow  513  for coordinates from which an image is acquired third, and the focusing is performed. The Z coordinate at which the focusing is completed is indicated by a star sign  523 . Next, the Z coordinate is moved in the range indicated by the arrow  514  for coordinates from which an image is acquired fourth, and the focusing is performed. The Z coordinate at which the focusing is completed is indicated by a star sign  524 . In the schematic diagram, the first to fourth image acquisition coordinates are initial coordinate groups. 
         [0068]    Next, a curved surface is approximated using the first to fourth image acquisition coordinates and the Z coordinates indicated by the star signs  521 ,  522 ,  523 , and  524 . The Z coordinate on the curved surface approximated for the fifth image acquisition coordinates or the Z coordinate at which the focusing is estimated to be completed for the fifth image acquisition coordinates is estimated. Next, a range of an arrow  515  is determined for the fifth image acquisition coordinates, the Z coordinate is moved to the determined range, and the focusing is performed. The center of the arrow  515  is a Z coordinate of image acquisition coordinates corresponding to the approximated curved surface. The Z coordinate to which the focal point is adjusted on the image acquisition coordinates is indicated by a triangle  525 . 
         [0069]    Next, a curved surface is approximated using the first to fifth image acquisition coordinates and the Z coordinates indicated by the star signs  521 ,  522 ,  523 , and  524  and the triangle  525 . A range of an arrow  546  is determined for the sixth image acquisition coordinates, the Z coordinate is moved to the determined range, and the focusing is performed. The Z coordinate at which the focusing is completed is indicated by a triangle  526 . Next, a curved surface is approximated using the first to sixth image acquisition coordinates and the Z coordinates indicated by the star signs  521 ,  522 ,  523 , and  524  and the triangles  525  and  526 . A range of an arrow  547  is determined for the seventh image acquisition coordinates, the Z coordinate is moved to the determined range, and the focusing is performed. The Z coordinate at which the focusing is completed is indicated by a triangle  527 . Next, a curved surface is approximated using the first to seventh image acquisition coordinates and the Z coordinates indicated by the star signs  521 ,  522 ,  523 , and  524  and the triangles  525 ,  526  and  527 . A range of an arrow  548  is determined for the eighth image acquisition coordinates, the Z coordinate is moved to the determined range, and the focusing is performed. The Z coordinate at which the focusing is completed is indicated by a triangle  528 . Next, a curved surface is approximated using the first to seventh image acquisition coordinates and the Z coordinates indicated by the star signs  521 ,  522 ,  523 , and  524  and the triangles  525 ,  526 ,  527 , and  528 . A range of an arrow  549  is determined for the ninth image acquisition coordinates, the Z coordinate is moved to the determined range, and the focusing is performed. A Z coordinate at which the focusing is completed is indicated by a triangle  529 . 
         [0070]    Unlike  FIG. 2 , according to the image acquisition program described with reference to  FIG. 12 , curved surfaces are approximated using all information about Z coordinates to which the focal point have been adjusted and from which images are already acquired, before an image is acquired from new coordinates. Thus, the latter half of generating the focus map, the higher the accuracy of a focus map and the smaller the scanning range of the Z coordinate for performing the focusing. The advantageous is more remarkable as the portions to be imaged increase in number, and focusing can be achieved in a shorter time. 
         [0071]    In the aforementioned embodiments, the focus map is updated every time the process is performed. The focus map, however, may be updated every time the process is performed a plurality of times such as 10 times or 20 times. 
         [0072]    The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiment is therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims, rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. 
       DESCRIPTION OF THE CODES 
       [0073]      201  . . . electron source  202  . . . acceleration electrode  203  . . . focusing lens  204  . . . deflector  205  . . . objective lens  206  . . . substrate (wafer)  207  . . . driving stage  208  . . . primary electron  209  . . . secondary electron  210  . . . detector  211  . . . A/D converter  213  . . . overall controller  214  . . . CPU  215  . . . primary storage device  216  . . . secondary storage device  217  . . . output unit  218  . . . bus  219  . . . reflected electron  220   a , 220   b  . . . reflected electron detector  320  . . . substrate  321  . . . notch  501  . . . surface of the substrate