Abstract:
When the lengths of FEM wafers are automatically measured, not only the sizes of targets, the lengths of which are to be measured, are often varied from those in registration, but also the patterns of the targets are often deformed. Therefore, it is difficult to automatically determine whether the length measurement is possible or not. Therefore, the following are executed with a semiconductor inspection system: (1) a process of identifying the position of the contour line of an inspected image using a distance image calculated from a reference image, (2) a process of calculating a defect size image based on the position of the contour line with respect to the identified distance image, and detecting a defect candidate from the defect size image, and (3-1) a process of, upon detection of the defect candidate, calculating the size of the detected defect candidate, or (3-2) a process of detecting a portion different between the first and second contour lines as the defect candidate.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a semiconductor inspection system that inspects or measures semiconductor circuit patterns using charged particle beams. 
       BACKGROUND ART 
       [0002]    With a reduction in the size of semiconductor devices (i.e., semiconductor integrated circuits), it has become difficult to form circuit structures in ideal shapes. Therefore, exposure simulation and circuit designing through optical proximity correction (OPC) have become of increasing importance. In the following description, the “optical proximity correction” is abbreviated to “OPC.” The “exposure simulation” means an optical simulation of an exposure step that is conducted in forming a semiconductor circuit on a substrate. The “designing through OPC” means circuit designing that is conducted by taking into consideration the manufacturing conditions, in particular, the exposure conditions of semiconductors. 
         [0003]    For both the “exposure simulation” and circuit “designing through OPC,” it is necessary to inspect the optical properties of an exposure system in advance. During the inspection conducted in advance, representative values, such as the dimensions and shape, of the actually produced semiconductor structure are measured using a length-measuring scanning electron microscope. Usually, measurement procedures of a length-measuring scanning electron microscope are registered in a file called a recipe in advance, so that the length-measuring scanning electron microscope executes automatic measurement in accordance with the recipe. 
         [0004]    By the way, automatic measurement of a FEM (Focus Exposure Matrix) wafer, which is conducted using a length-measuring scanning electron microscope to inspect the optical properties of an exposure system, has the following problems. 
         [0005]    A first problem is that the size of a semiconductor structure to be measured differs depending on the exposure conditions. When identical semiconductor structures are produced under a plurality of different exposure conditions, the resulting semiconductor structures will have different widths and heights. In the automatic measurement, if the size of a structure to be measured is different from that registered in a recipe, it would be difficult to execute position collation (i.e., pattern matching) through image processing. 
         [0006]    The second problem is that if a structure to be measured is not formed, erroneous measurement may be executed. In order to inspect the optical properties of an exposure system, a semiconductor structure is produced in the stage where the exposure conditions are not determined yet. Therefore, there may be cases where semiconductor structures cannot be formed depending on the exposure conditions used. Usually, a length-measuring scanning electron microscope captures an image of a structure to be measured, and outputs the dimensions of the structure calculated through software processing of the captured image. Because of such a mechanism, the length-measuring scanning electron microscope calculates values even when there is no structure formed in the image-capturing region. That is, the length-measuring scanning electron microscope calculates erroneous dimensions as the measured values. 
         [0007]    Solutions to the first problem are described in Patent Literatures 1 and 2, for example. Patent Literatures 1 and 2 each disclose a method of allowing pattern matching to be performed on measurement targets with different sizes. A solution to the second problem is described in Patent Literature 3, for example. According to such patent literature, when a shape deformation is detected, pattern matching is determined to fail, and thus, execution of measurement is prohibited. 
       CITATION LIST 
     Patent Literature 
       [0000]    
       
         Patent Literature 1: JP 2007-79982A 
         Patent Literature 2: JP 2007-256225 A 
         Patent Literature 3: JP 2009-223414 B 
       
     
       Non Patent Literature 
       [0000]    
       
         Non Patent Literature 1: F. Y. Shih, Y.-T. Wu: “Fast Euclidean distance transformation in two scans using a 3×3 neighborhood,” Computer Vision Image Understanding, vol. 93, 2004, p 195-205 
       
     
       SUMMARY OF INVENTION 
     Technical Problem 
       [0012]    When the first problem and the second problem occur individually, such problems can be solved with the aforementioned solutions. However, when the first problem and the second problem occur concurrently, it would be impossible to solve the two problems concurrently with any of the aforementioned solutions. 
         [0013]    In practice, in the production of semiconductor structures, when structures are produced under variously changed exposure conditions, and the dimensions of the thus produced structures are automatically measured, the sizes of the structures to be measured may differ depending on the exposure conditions, and there may be even cases where structures are not formed. 
         [0014]    When the presence or absence of a target to be measured is not known in advance, a length-measuring scanning electron microscope is required to have a mechanism for determining whether measurement is executable or not depending on the presence or absence of a structure formed. Specifically, the length-measuring scanning electron microscope is required to have a mechanism for executing measurement when a structure is formed and not executing measurement when a structure is not formed. In other words, the length-measuring scanning electron microscope is required to have a tolerance to variations in size and have a mechanism capable of determining the presence or absence of a structure formed and also determining whether measurement is executable or not. However, with the existing solutions, it would be impossible to determine whether measurement is executable or not when a produced semiconductor has a changed size due to deformation as described above. 
         [0015]    The inventor has conducted concentrated studies about the aforementioned technical problems, and arrived at an invention indicated below. 
       Solution to Problem 
       [0016]    The inventor provides as an invention an inspection technique that is sensitive to a defect size. The invention includes (1) a process of identifying the position of the contour line of an inspected image using a distance image calculated from a reference image, (2) a process of calculating a defect size image based on the position of the contour line with respect to the identified distance image, and detecting a defect candidate from the defect size image, and (3) a process of, upon detection of the defect candidate, calculating the size of the detected defect candidate. 
         [0017]    The inventor also proposes as an invention an inspection technique that is sensitive to the size of the contour of a defect. The invention includes (1) a process of extracting a first contour line from a reference image, (2) a process of extracting a second contour line from the detected image, and (3) a process of detecting a portion different between the first and second contour lines as a defect candidate. 
       Advantageous Effects of Invention 
       [0018]    According to the present invention, defect detection that is sensitive to the size of a defect or the contour thereof can be realized. Other problems, structures, and advantageous effects will become apparent from the following description of embodiments. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0019]      FIG. 1  is a view showing an exemplary schematic configuration of a semiconductor inspection system. 
           [0020]      FIG. 2  is a view illustrating an inspected image contour line, a reference contour line image, and a reference distance image. 
           [0021]      FIG. 3  is a view illustrating collation between an inspected image contour line and a reference distance image. 
           [0022]      FIG. 4  is a view showing a defect size image created through collation between an inspected image contour line and a reference distance image. 
           [0023]      FIG. 5  is a view illustrating another method for creating a defect size image. 
           [0024]      FIG. 6  is a view illustrating a defect extraction image that is created from a defect size image. 
           [0025]      FIG. 7  is a view illustrating a method for detecting the size of the contour of a defect. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0026]    Hereinafter, embodiments of the present invention will be described with reference to the drawings. It should be noted that embodiments of the present invention are not limited to those described below, and various variations are possible within the spirit and scope of the present invention. 
       Embodiment 1 
     System Configuration 
       [0027]      FIG. 1  shows an exemplary schematic configuration of a defect inspection system that is an exemplary semiconductor inspection system in accordance with an embodiment. This embodiment will describe a case where an inspected image is acquired using an electron beam. That is, a case where an electron microscope  101  is used for inspection of a sample  106  will be described. Needless to say, an inspected image can also be acquired using an ion beam. Hereinafter, the device configuration that uses the electron microscope  101  will be described. 
         [0028]    The electron microscope  101  has arranged therein an electron gun  102  as an electron source. An electron beam  103  emitted from the electron gun  102  is converged by an electron lens  104  and is caused to irradiate the surface of the sample  106 . A secondary signal  109 , such as secondary electrons and reflected electrons, is generated from a position irradiated with the electron beam  103 . The secondary signal  109  is detected by an electron detector  110 , and is converted into an intensity signal  111  representing the intensity of the signal. Herein, deflection of the electron beam  103  is controlled by a deflecting magnetic field generated by a deflector  105 . By such deflection control, the position irradiated with the electron beam  103  on the sample  106  is raster-scanned. Raster scan with the electron beam  103  is controlled by a control signal  112  generated by a control system  113 . 
         [0029]    The intensity signal  111  is provided to an image processing processor  114 , and is converted into a digital signal by an A/D converter circuit (not shown). The intensity signal that has been converted into a digital signal is provided to a digital image creation unit  115 . The digital image creation unit  115  generates an inspected electron microscope image  127  from the provided digital signal. The thus generated inspected electron microscope image  127  is displayed on an operation screen  126  of a display device  125 . The operation screen  126  displays an image field to be checked by an operator and a character/numerical value entry field. Characters and numerical values are input via an operating device  134 . 
         [0030]    It should be noted that the sample  106  is mounted on a sample table  107 , and the position and tilt of the sample table  107  with respect to the electron beam  103  are drive-controlled by a sample table control device  108 . Such drive control allows a region irradiated with the electron beam  103  (i.e., a raster-scanned range) to be movable on the sample  106 . 
         [0031]    The control system  113  controls imaging and inspection of the sample  106  (e.g., a semiconductor wafer) performed by the electron microscope  101 , based on information on the region to be inspected. The control system  113  communicates with the image processing processor  114  to exchange information necessary for the processing operation. The control system  113  inputs characters or numerical values, which have been entered via the operation screen  126 , via the display device  125 . It should be noted that each of the electron microscope  101 , the control system  113 , the image processing processor  114 , the display device  125 , and the reference data storage system  117  has a communication device (not shown) used for exchanging data. 
         [0032]    The image processing processor  114  includes an arithmetic unit and a storage unit (i.e., ROM and RAM) (not shown), and provides processing functions indicated below through processing of programs. It should be noted that each processing function may be partially or entirely implemented as hardware. 
         [0033]    The image matching unit  137  is a processing unit that executes alignment between an inspected image and a reference image. The inspected image herein corresponds to the inspected electron microscope image  127  generated by the digital image processing unit  115 . The reference image is provided as reference data input from the reference data storage system  117 . For the reference data, one of circuit design data, reticle pattern data for exposure, an exposure simulation result, an electron microscope image, or the like is used. When circuit data is used as the reference data, the image matching unit  137  performs alignment between the inspected image and the reference image using the existing technology disclosed in Patent Literature 1 or 2. 
         [0034]    An inspected image contour line extraction unit  116  creates an inspected image contour line  135  corresponding to the outline of a structure  132  from the inspected electron microscope image  127 . In the operation screen  126  in  FIG. 1 , the inspected image contour line  135  is represented by a dotted line. 
         [0035]    A reference image contour line extraction unit  118  creates a reference image contour line  130  corresponding to the outline of the reference image. In the operation screen  126  in  FIG. 1 , the reference image contour line  130  is represented by a dashed line. 
         [0036]    A distance image conversion unit  119  creates a reference distance image (described below) from the reference image contour line  130 . For creation of the reference distance image, a known distance image creation method is used. Examples of the known distance image creation method include a method described in Non Patent Literature 1. In this embodiment, as the reference distance image, information on the distance from the reference image contour line  130  is defined as the luminance value of the image. Thus, pixel rows each having identical luminance values are arranged concentrically with respect to the reference image contour line  130 . 
         [0037]    A collation unit  120  collates the inspected image contour line  135  with the reference distance image, and generates at the pixel points on the inspected image contour line  135  a defect size image including distance information. During the collation, the collation unit  120  acquires from the image matching unit  137  positional information in which the inspected image matches the reference image. In this embodiment, two methods are prepared as a method for generating a defect size image, and which of the two methods should be applied can be instructed via the operation screen  126 . 
         [0038]    The defect candidate detection unit  121  applies a blob detection process to the defect size image to detect a defect candidate. The term “blob” means a group of pixels having identical luminance values. In this embodiment, a luminance value is given as information on the distance from the reference image contour line  130 . The defect candidate detection unit  121  extracts a group of pixels whose luminance values are greater than a determination threshold as a candidate region of defects, and sets the group as a defect extraction image. The defect extraction image is provided to a defect size determination unit  122 , a defect position determination unit  123 , and a unit  124  for determining defects/determining if measurement is possible. 
         [0039]    The defect size determination unit  122  determines information on the size of the candidate region. In this embodiment, the area of the candidate region is calculated as the size. The calculated area is provided as the size information to the unit  124  for determining defects/determining if measurement is possible. 
         [0040]    The defect position determination unit  123  determines the position of the candidate region. That is, the defect position determination unit  123  determines the position of a defect region  131  (e.g., the maximum distance from the reference image contour line  130 ). The determined positional information is provided to the unit  124  for determining defects/determining if measurement is possible. 
         [0041]    The unit  124  for determining defects/determining if measurement is possible determines if the candidate region is defective or not based on the size information and/or the positional information. For example, the unit  124  for determining defects/determining if measurement is possible compares the size information (i.e., the area of the candidate region) with a determination threshold, and determines the candidate region to be defective if the size information thereof is greater than the determination threshold, and determines the candidate region to be not defective if the size information thereof is less than the determination threshold. In addition, the unit  124  for determining defects/determining if measurement is possible compares the positional information with a determination threshold, and determines the candidate region to be defective if it includes a pixel whose distance is greater than the determination threshold, and determines the candidate region to be not defective if it includes only pixels whose distances are less than the determination threshold. 
         [0042]    The determination threshold herein is given as an input value to the operation screen  126 . Needless to say, the initial value may also be used. In addition, a determination result for each candidate region is output to the operation screen  126 . It should be noted that the unit  124  for determining defects/determining if measurement is possible determines if measurement (e.g., length measurement) is executable based on the determination of the presence or absence of defects. Information about if measurement is executable is output to a length-measuring scanning electron microscope (not shown) from the unit  124  for determining defects/determining if measurement is possible. It should be noted that the length-measuring scanning electron microscope may be the electron microscope  101 . When execution of measurement is permitted, the length-measuring scanning electron microscope reads a recipe from a storage region (not shown), and executes an automatic length-measuring operation. 
         [0043]    On the operation screen  126  shown in  FIG. 1 , the inspected electron microscope image  127  of the sample  106 , the reference image contour line  130  (dashed line), the defect region  131  (broken line), and the inspected image contour line  135  (dotted line) are displayed. Besides, a defect dimension threshold entry field  128 , a defect area threshold entry field  133 , a defect list  136 , and a mode switching entry field  140  are displayed on the operation screen  126 . 
         [0044]    A determination threshold (i.e., a dimension value) to be used by the defect position determination unit  123  is input to the defect dimension threshold entry field  128 . In  FIG. 1 , “5.0 nm” is input as a threshold  138 . A determination threshold (i.e., a dimension value) to be used by the defect size determination unit  122  is input to the defect area threshold entry field  133 . In  FIG. 1 , “40.0 nm 2 ” is input as a threshold  139 . Such numerical values are input via the operation device  134 . Although  FIG. 1  represents a case where the operation device  134  is a keyboard, other input devices, such as a mouse and a stylus pen, may also be used. 
         [0045]    The defect list  136  displays information about a plurality of candidate regions that have been determined to be defective by the defect size determination unit  122  and/or the defect position determination unit  123 . In the defect list  136 , the number that identifies a defect is associated with positional information and size information. In the mode switching entry field  140 , a button in a toggle switch form is displayed to allow one of the two methods for creating a defect size image (described below) to be selectable. Herein, the two creation methods are indicated by “Mode 1” and “Mode 2.” 
       [Operation of Extracting Inspected Image Contour Line and Operation of Creating Reference Distance Image] 
       [0046]      FIG. 2  shows the content of the processing operations executed by the inspected image contour line extraction unit  116  and the distance image conversion unit  119 . 
         [0047]    The inspected image contour line extraction unit  116  extracts an inspected image contour line  204  corresponding to the outline of a structure  202  from the inspected electron microscope image  201  and creates an inspected contour line image  203 . Herein, as a method for extracting the inspected image contour line  204  from the inspected electron microscope image  201 , various known methods can be applied. For example, it is possible to apply a method of smoothing the inspected electron microscope image  201  using a Gaussian filter, and thereafter emphasizing the contour of the structure  202  using a secondary differentiation filter, and further binarizing the luminance value of the image obtained after emphasizing the contour. As a result of such process, the inspected contour line image  203  with the extracted inspected image contour line  204  is created. It should be noted that when there is no structure created, the inspected image contour line  204  is not extracted, or a small inspected image contour line  204  is extracted. 
         [0048]    Meanwhile, the distance image conversion unit  119  inputs a reference contour line image  205 , which has been obtained by converting a reference image contour line  206  into an image, from the reference image contour line extraction unit  119 . As described above, examples of the reference data for providing a reference image include circuit design data, reticle pattern data for exposure, an exposure simulation result, and an electron microscope image. Each of the circuit design data, the reticle pattern data for exposure, and the exposure simulation result is typically contour line data formed by a group of vertices. Thus, it is obvious that such data is used as the reference contour line image  205 . When an electron microscope image is used, it is possible to convert reference data into the reference contour line image  205  with the extracted reference image contour line  206  using the same method as a method for converting the inspected electron microscope image  201  into the inspected contour line image  203 . 
         [0049]    The distance image conversion unit  119  creates a reference distance image  207  by applying a distance image conversion process to the reference image contour line  206  of the reference contour line image  205 . As described above, the distance image conversion unit  119  executes a distance image conversion process by using a commonly known method such as the one described in Non Patent Literature 1. In the reference distance image  207 , the luminance value of the image is higher at a position farther from the reference image contour line  206 . 
         [0050]    In this specification, a region of the reference distance image  207  that overlaps the reference image contour line  206  is referred to as a “distance zero region”  208 . This indicates that the distance of pixels in this region from the reference image contour line  206  is less than a unit distance. Further, in this specification, regions that expand concentrically outward from the reference image contour line  206  are referred to as a “distance 1 region”  209 , a “distance 2 region”  210 , a “distance 3 region”  211 , and a “distance 4 region”  212  in this order. The “distance 1 region”  209  indicates that the distance of pixels in this region from the reference image contour line  206  is not less than a unit distance and less than 2 units. The same holds true for the other regions. 
         [0051]    It should be noted that regions with different luminance values are also set in a region  213 , which is the outer region of the “distance 4 region”  212 , in accordance with the distance from the reference image contour line  206 . However, such regions are omitted in  FIG. 2  for the sake of convenience. In addition, a “distance 1 region”  214 , a “distance 2 region”  215 , and a “distance 3 region”  216  are also arranged concentrically on the inner side of the reference image contour line  206 . 
         [0052]    In this specification, all of the pixels located within the “distance zero region”  208  have a luminance value of “zero.” All of the pixels located within the “distance 1 region”  209  and the “distance 1 region”  214  have a luminance value of “1.” All of the pixels located within the “distance 2 region”  210  and the “distance 2 region”  215  have a luminance value of “2.” All of the pixels located within the “distance 3 region”  211  and the “distance 3 region”  216  have a luminance value of “3.” All of the pixels located within the “distance 4 region”  212  have a luminance value of “4.” The pixels located in the region  213  have different luminance values depending on the distance from the reference image contour line  206 . Herein, such pixels are omitted for the sake of convenience. 
         [0053]    In this embodiment, the luminance value is set using a specific unit, such as a gray scale value (i.e., gray level), a reduced scale value (e.g., units of nm) of the actual dimension of the imaging target, or the distance of a pixel within the image (e.g., pixels). 
       [Collation Operation] 
       [0054]      FIG. 3  shows the content of the operation of the collation unit  120  to collate the inspected image contour line  135  with the reference distance image  207 . A defect size image is created through such a process. In  FIG. 3 , an inspected image contour line  311  is overlaid on a reference distance image  301 . For overlaying, information for aligning the inspected image with the reference image from the image matching unit  137  is used. It should be noted that the reference distance image  301  has the same structure as that illustrated in  FIG. 2 .  FIG. 3  shows an enlarged view of  FIG. 2  to illustrate the detailed collation operation. 
         [0055]    The reference distance image  301  includes a “distance zero region”  302 , a “distance 1 region”  306 , a “distance 2 region”  307 , a “distance 3 region”  308 , a “distance 4 region”  309 , a “region”  310 , a “distance 1 region”  303 , a “distance 2 region”  304 , and a “distance 3 region”  305 . Regions with different luminance values are also set sequentially in the “region”  310  in accordance with the distance from the reference image contour line. However, such regions are omitted in  FIG. 3 . 
         [0056]      FIG. 3  represents some of the pixels that form the inspected image contour line  135 , that is, pixels  312 ,  313 ,  314 ,  315 ,  316 ,  317 ,  318 ,  319 ,  320 ,  321 ,  322 ,  323 ,  324 ,  325 ,  326 ,  327 ,  328 ,  329 ,  330 , and  331 . Needless to say, a number of pixels other than these are also present on the inspected image contour line  311 . In  FIG. 3 , such pixels are omitted for the sake of convenience. 
         [0057]    When the inspected image contour line  311  is overlaid on the reference distance image  301 , the pixels  312 ,  313 ,  314 ,  315 ,  316 ,  317 ,  318 ,  319 ,  325 ,  326 ,  327 ,  328 ,  329 ,  330 , and  331  are located on the “distance 2 region”  307 . The pixels  320  and  324  are located on the “distance 3 region”  308 . The pixels  321 ,  322 , and  323  are located on the “distance 4 region”  309 . 
       [Example 1 of Creation of Defect Size Image] 
       [0058]      FIG. 4  exemplarily shows a method for creating a defect size image  401  as a result of the collation operation of the collation unit  120 . In the defect size image  401 , each pixel that forms the inspected image contour line  311  is associated with a luminance value of a corresponding region on the reference distance image  301 . In  FIG. 4 , a difference in luminance value is represented by a gray level. 
         [0059]    The collation unit  120  converts the pixels  312 ,  313 ,  314 ,  315 ,  316 ,  317 ,  318 ,  319 ,  325 ,  326 ,  327 ,  328 ,  329 ,  330 , and  331  on the “distance 2 region”  307  in  FIG. 3  into pixels  403 ,  404 ,  410 ,  411 ,  412 ,  413 ,  414 ,  415 ,  416 ,  417 ,  418 ,  419 ,  420 ,  421 ,  422 ,  423 ,  424 ,  425 ,  426 ,  427 ,  428 ,  429 ,  430 ,  431 ,  432 ,  433 ,  434 ,  435 ,  436 ,  437 ,  438 ,  439 , and  440  on the defect size image  401 . The pixels after the conversion have an unchanged luminance value of “2” from that of the “distance 2 region”  307 . 
         [0060]    Likewise, the collation unit  120  converts the pixels  320  and  324  on the “distance 3 region”  308  in  FIG. 3  into pixels  405  and  409  on the defect size image  401 . The pixels after the conversion have an unchanged luminance value of “3” from that of the “distance 3 region.” 
         [0061]    Although  FIG. 4  shows only representative pixels that partially form the inspected image contour line  402  for the sake of convenience, all of the pixels on the inspected image contour line  402  have the luminance values on the reference distance image  301 . The range of the luminance values of the pixels on the inspected image contour line  402  is zero to the distance of the diagonal line of the image. In this embodiment, “zero,” a value that is less than or equal to “−1,” or other specific luminance values are given to pixels in an outer region  441  of the inspected image contour line  402 . Herein, as the specific given values, a value outside the range of the luminance values given to the inspected image contour line  402 , and the like are considered. 
       [Example 2 of Creation of Defect Size Image] 
       [0062]      FIG. 5  exemplarily shows another method for creating the defect size image  401  as a result of the collation operation of the collation unit  120 . That is,  FIG. 5  shows a method different from that in  FIG. 4 . It should be noted that the method described with reference to  FIG. 4  shows a case where pixels that form the inspected image contour line  311  have unchanged luminance values at positions where the pixels overlap the reference distance image  301 . 
         [0063]    Meanwhile, the method described with reference to  FIG. 5  uses the luminance values of the reference distance image  301  ( FIG. 3 ), a tilt  514  of the reference distance image  301 , and a direction  513  of the tangent to the inspected image contour line  311  ( FIG. 3 ). 
         [0064]    An enlarged view  501  is a partially enlarged view of the reference distance image  301 . The enlarged view  501  includes a “distance zero region”  517  having a luminance value of “zero,” a “distance 1 region”  502  and a “distance 1 region”  503  each having a luminance value of “1,” a “distance 2 region”  504  having a luminance value of “2”, a “distance 3 region”  505  having a luminance of “3,” a “distance 4 region”  506  having a luminance value of “4,” and a “distance 5 region”  507  having a luminance value of “5.” In addition, the pixels  508 ,  509 , and  510  are adjacent pixels on the inspected image contour line  311 . 
         [0065]    In  FIG. 5 , an arrow indicating the direction  513  of the tangent to the pixel  508  is depicted. The pixels  510  and  508  are adjacent pixels. Therefore, a direction  511  from the pixel  510  to the adjacent pixel (i.e., the pixel  508 ) can be easily calculated from the coordinates of the pixels. Likewise, the pixels  508  and  509  are adjacent pixels. Therefore, a direction  512  from the pixel  508  to the adjacent pixel (i.e., the pixel  509 ) can also be easily calculated. In this embodiment, the direction  513  of the tangent to the pixel  508  is calculated as the mean vector of the direction  511  and the direction  512 . 
         [0066]    The tilt  514  of the reference distance image  301  is given by the direction of the tilt of the luminance values of the reference distance image  301 , and is a direction in which the luminance value increases. In the enlarged view  501 , the tilt  514  is calculated from the luminance values of a region including three pixels×three pixels, with the luminance value of a pixel that is adjacent to the pixel  508  as the center. A method for calculating the tilt is a common method of the image processing. 
         [0067]    A contour line direction  515  is given as a counterclockwise direction of 90° with respect to the tilt  514  of the reference distance image  301 . An angle  516  is calculated from the contour line direction  515  and the tangential direction  513 . In  FIG. 5 , the angle  516  is 60°. 
         [0068]    In this case, the luminance value of each pixel that forms the defect size image is determined as follows. As described above, in this embodiment, the pixel  508  is located in the “distance 3 region.” Thus, the luminance value of the pixel  508  is “3.” In addition, the angle  516  is 60°. At this time, the luminance value corresponding to the pixel  508  on the reference distance image  301  is given by the following formula. 
         [0000]      3×cos(60°)=3×√3/2
 
         [0069]    This computation method can be generalized as can be seen in the following formula for any pixel of the inspected image contour line  311  and the reference distance image  301 . 
         [0000]        y=x ·cos θ
 
         [0070]    It should be noted that symbol x represents the luminance value of the reference distance image  301 , θ represents the tilt of the reference distance image  301 , and y represents the luminance value of the defect size image. The computation method herein is a method of modifying the luminance value at a position that overlaps the reference distance image  301  using a weight that reflects a local tilt of each pixel. Incidentally, when the tangential direction  513  coincides with the contour line direction  515  (θ=0°), the luminance value of the defect size image coincides with that shown in  FIG. 4 . Meanwhile, when the tangential direction  513  coincides with the tilt  514  of the reference distance image  301  (θ=90°), the luminance value of the defect size image becomes zero. 
         [0071]    In this specification, a method for directly creating the defect size image  401  from the luminance value of the reference distance image  301  will be referred to as “Mode 1,” and a method for creating the defect size image  401  using the luminance value of the reference distance image  301 , the tilt  514  of the reference distance image  301 , and the direction  513  of the tangent to the inspected image contour line  311  will be referred to as “Mode 2.” The processing time of “Mode 1” can be shorter than that of “Mode 2” since “Mode 1” involves a smaller number of processing steps. Meanwhile, the area calculation accuracy of “Mode 1” is lower than that of “Mode 2.” 
         [0072]    It should be noted that the area calculation accuracy of “Mode 2” is higher than that of “Mode 1.” However, “Mode 2” involves a greater number of processing steps than “Mode 1.” This is the reason why the processing time of “Mode 2” is longer than that of “Mode 1.” 
         [0073]    As described above, each of “Mode 1” and “Mode 2” has both advantages and disadvantages. Therefore, it is desirable that “Mode 1” and “Mode 2” be selectively used in accordance with the intended use. In addition, as described above, a mode used by the collation unit  120  can be switched upon input of an operation to the mode switching entry field  140  of the operation screen  126 . 
       [Detection of Defect Region] 
       [0074]      FIG. 6  shows the content of the operation of the defect candidate detection unit  121  to detect the defect region  131 . The defect candidate detection unit  121  creates from the defect size image  401  a defect extraction image  601  including only the defect region  131  in accordance with the following procedures. 
         [0075]      FIG. 6  represents an inspected image contour line  602  and a reference image contour line  609  for the sake of convenience. It should be noted, however, that these lines are shown only supplementarily to clarify the positional relationship of the defect region  615 , and are not displayed in the actual defect extraction image  601 . 
         [0076]      FIG. 6  shows pixels  603 ,  604 ,  605 ,  606 , and  607  on the contour line of the defect extraction image  601 . A distance  610  shown by an arrow in  FIG. 6  is the distance between the pixel  603  and the reference image contour line  609 . A distance  611  shown by an arrow in  FIG. 6  is the distance between the pixel  604  and the reference image contour line  609 . A distance  612  shown by an arrow in  FIG. 6  is the distance between the pixel  605  and the reference image contour line  609 . A distance  613  shown by an arrow in  FIG. 6  is the distance between the pixel  606  and the reference image contour line  609 . A distance  614  shown by an arrow in  FIG. 6  is the distance between the pixel  607  and the reference image contour line  609 . It should be noted that the defect region  615  shown by a colored region represents a region of defects displayed in the defect extraction image  601 . 
         [0077]    Herein, each arrow that indicates the distance  610 ,  611 ,  612 ,  613 , or  614  and the defect region  615  are shown for the sake of convenience, and are not present in the actual defect extraction image  601 . 
         [0078]    First, the defect candidate detection unit  121  compares each pixel of the defect size image  401  with the threshold  138  input to the defect dimension threshold entry field  128 , and extracts only pixels whose values are greater than the threshold. It should be noted that the threshold  138  has been converted to conform to the reduced scale of the defect size image  401 . 
         [0079]    For example, in the reduced scale of the defect size image  401 , when the threshold  138  is “Distance 2,” the defect candidate detection unit  121  determines a group of pixels having distance information that is greater than or equal to “Distance 3” to be the defect region  615 . In such a case, the defect candidate detection unit  121  extracts pixels whose luminance values are greater than or equal to three from among the pixels on the inspected image contour line  402  in  FIG. 4 . That is, the defect candidate detection unit  121  extracts the pixels  405 ,  406 ,  407 ,  408 , and  409 . Such pixels correspond to the pixels  603 ,  604 ,  605 ,  606 , and  607  of the defect extraction image  601 . The created defect extraction image  601  is provided to the defect size determination unit  122 , the defect position determination unit  123 , and the unit  124  for determining defects/determining if measurement is possible. 
         [0080]    Next, the defect size determination unit  122  determines the sum total of the luminance values of the continuous pixels in the defect extraction image  601 . In  FIG. 6 , the pixels  603 ,  604 ,  605 ,  606 , and  607  are the continuous pixels. 
         [0081]    In this embodiment, the luminance value of the pixel  603  is “3,” the luminance value of the pixel  604  is “4,” the luminance value of the pixel  605  is “4,” the luminance value of the pixel  606  is “4,” and the luminance value of the pixel  607  is “3.” Thus, the sum total of such pixels is 18 (=3+4+4+4+3). 
         [0082]    The luminance value of the pixel  603  and the distance  610  are the same, the luminance value of the pixel  604  and the distance  611  are the same, the luminance value of the pixel  605  and the distance  612  are the same, the luminance value of the pixel  606  and the distance  613  are the same, and the luminance of the pixel  607  and the distance  614  are the same. Further, the pixels  603 ,  604 ,  605 ,  606 , and  607  are away by a distance of “1” in the contour line direction. Herein, the sum total of the luminance values of the pixels  603 ,  604 ,  605 ,  606 , and  607  coincides with the area of the defect region  615 . 
         [0083]    The unit  124  for determining defects/determining if measurement is possible compares the calculated area of the defect region  615  with the threshold  139  input to the defect area threshold entry field  133 . Herein, when the area of the defect region  615  is greater than the threshold  139 , the unit  124  for determining defects/determining if measurement is possible determines that the defect region  615  is defective. Meanwhile, when the area of the defect region  615  is smaller than the threshold  139 , the unit  124  for determining defects/determining if measurement is possible determines that the defect region  615  is not defective. It should be noted that the threshold  139  has been converted to conform to the reduced scale of the defect size image  401 . Herein, when the area of the candidate region  615  is greater than or equal to the threshold  139 , the area of the entire candidate region  615  is the area of the defect. 
         [0084]    Although this embodiment illustrates a case where whether the candidate region  615  is defective or not is determined based only on the area of the candidate region  615 , it is also possible to determine whether the candidate region  615  is defective or not based on the distance of (positional information on) the candidate region  615  from the reference image contour line  609 . For example, when the maximum value of the luminance values of the pixels that form the defect extraction image  601  is greater than the threshold  138 , the candidate region  615  is determined to be defective. It is also possible to determine whether the candidate region  615  is defective or not based on both the area of and the positional information on the candidate region  615 . 
         [0085]    Upon detection of a defect, the unit  124  for determining defects/determining if measurement is possible prohibits the execution of the length-measuring operation of the length-measuring scanning electron microscope. Meanwhile, when a defect is not detected, the unit  124  for determining defects/determining if measurement is possible continuously permits the execution of the length-measuring operation of the length-measuring scanning electron microscope. 
       Conclusion 
       [0086]    As described above, when the defect inspection system in accordance with this embodiment is used, it is possible to realize defect detection that is sensitive to a defect size in a shorter processing time. In addition, when the defect inspection system in accordance with this embodiment is used, it is possible to calculate a defect size through comparison with a reference image as the standard data (i.e., reference data). Further, when the defect inspection system in accordance with this embodiment is used, it is possible to identify the position of the detected defect on the image. Further, when the defect inspection system in accordance with this embodiment is used, it is possible to control the detection of a defect in accordance with the size and/or the position of the candidate region  615 . Further, when the defect inspection system in accordance with this embodiment is used, it is possible to visually check the position and the size of a defect on the image on the operation screen  126 . Further, when the defect inspection system in accordance with this embodiment is used, it is possible to automatically determine whether the following length measurement is executable or not in accordance with the presence or absence, size and/or position of a defect. Furthermore, when the defect inspection system in accordance with this embodiment is used, it is possible to provide the automatic determination result of the measurement to a user via the operation screen  126 . 
       Embodiment 2 
       [0087]    The previous embodiment has described a case where the defect extraction image  601  is created from the defect size image  401 , and then, the presence or absence of defects and whether the following measurement is executable or not are determined based on the size of the defect region  615  extracted from the defect extraction image  601 . 
         [0088]    However, as shown in  FIG. 7 , it is also possible to compare an inspected electron microscope image  701  with a reference contour line image  703 , create a differential image  705  of the inspected image contour line  702  and the reference image contour line  704 , and determine the presence or absence of defects as well as whether the following measurement is executable or not based on a differential contour line  706  corresponding to the difference between the two contour lines. 
         [0089]    Specifically, it is possible to determine the size and length of the differential contour line  706  from the differential image  705  and then determine the presence or absence of defects and whether the following measurement is executable or not by comparing each value with a corresponding threshold. It should be noted that the basic configuration of the defect inspection system in accordance with this embodiment may be the same as the device configuration shown in  FIG. 1 . 
       Other Embodiment 
       [0090]    The present invention is not limited to the aforementioned embodiments, and includes a variety of variations. For example, although the aforementioned embodiments have been described in detail to clearly illustrate the present invention, the present invention need not include all of the structures described in the embodiments. It is possible to replace a part of a structure of an embodiment with a structure of another embodiment. In addition, it is also possible to add, to a structure of an embodiment, a structure of another embodiment. Further, it is also possible to, for a part of a structure of each embodiment, add/remove/substitute a structure of another embodiment. 
         [0091]    Some or all of the aforementioned structures, functions, processing units, processing means, and the like may be implemented as an integrated circuit or other hardware, for example. Alternatively, each of the aforementioned structures, functions, and the like may be implemented through analysis and execution of a program that implements each function by a processor. That is, each of the aforementioned structures, functions, and the like may also be implemented as software. Information such as the program that implements each function, tables, and files can be stored in a storage device such as memory, a hard disk, or a SSD (Solid State Drive); or a storage medium such as an IC card, an SD card, or a DVD. 
         [0092]    In addition, the control lines and information lines represent those that are considered to be necessary for the description, and do not necessarily represent all control lines and information lines that are necessary for a product. Thus, in practice, almost all structures may be considered to be mutually connected. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           101  Electron microscope 
           102  Electron gun 
           103  Electron beam 
           104  Electron lens 
           105  Deflector 
           106  Sample 
           107  Sample table 
           108  Sample table control device 
           109  Secondary signal 
           110  Electron detector 
           111  Intensity signal 
           112  Control signal 
           113  Control system 
           114  Image processing processor 
           115  Digital image creation unit 
           116  Inspected image contour line extraction unit 
           117  Reference data storage system 
           118  Reference image contour line extraction unit 
           119  Distance image conversion unit 
           120  Collation unit 
           121  Defect candidate detection unit 
           122  Defect size determination unit 
           123  Defect position determination unit 
           124  Unit for determining defects/determining if measurement is possible 
           125  Display device 
           126  Operation screen 
           127  Inspected electron microscope image 
           128  Defect dimension threshold entry field 
           130  Reference image contour line 
           131  Defect region 
           132  Structure 
           133  Defect area threshold entry field 
           134  Operation device 
           135  Inspected image contour line 
           136  Defect list 
           137  Image matching unit 
           138  Threshold 
           139  Threshold 
           140  Mode switching entry field 
           201  Inspected electron microscope image 
           202  Structure 
           203  Inspected contour line image 
           204  Inspected image contour line 
           205  Reference contour line image 
           206  Reference image contour line 
           207  Reference distance image 
           301  Reference distance image 
           311  Inspected image contour line 
           401  Defect size image 
           402  Inspected image contour line 
           501  Enlarged view 
           511  Direction 
           512  Direction 
           513  Tangential direction 
           514  Tilt of reference distance image 
           515  Contour line direction 
           516  Angle 
           601  Defect extraction image 
           602  Inspected image contour line 
           609  Reference image contour line 
           615  Defect region 
           706  Differential contour line