Abstract:
An inspection method according to the embodiments includes applying light of a light source to an inspection target; receiving light from the inspection target to obtain a first image of the inspection target by a sensor; based on an image of a first pattern comprising repetitive patterns unresolvable with a wavelength of the light source in the first image, calculating a deviation of luminance values with respect to each of first regions in the first pattern by a processor; obtaining a second image of the inspection target by the sensor; correcting luminance values of the second image by the processor based on the deviations of the luminance values; and comparing the repetitive patterns of the corrected second image with each other by a comparer.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2016-112198, filed on Jun. 3, 2016, the entire contents of which are incorporated herein by reference. 
       FIELD 
       [0002]    The embodiments of the present invention relate to an inspection method and an inspection apparatus. 
       BACKGROUND 
       [0003]    The NIL (NanoImprint Lithography) technology has been developed as a technique that enables to form a microstructure of a semiconductor. In order to inspect a template of the NIL technology, an image of the template is taken with a SEM (Scanning Electron Microscope) and a shape defect such as a rupture or a bridge (short-circuit) in the microstructure or a critical defect such as a thickening or thinning of the line width is detected. Defect detection with a wafer inspection apparatus using a structure actually transferred on a wafer is also performed. 
         [0004]    However, in these methods, imaging takes a long time, or a detected defect cannot be always determined as a defect of the template because a result having subjected to a manufacturing process on a wafer is inspected. Therefore, these methods have a problem when used for the template inspection on a full scale. 
         [0005]    Meanwhile, trials of imaging a template with an inspection apparatus used in an inspection of a photomask and detecting a defect on the template based on the image have been conventionally performed (see Patent Literature 1, for example). In a current downscaled technology node, the dimension of the microstructure of a template is minuter than a light source wavelength of the inspection device and the inspection device cannot resolve the microstructure. 
         [0006]    That is, when an image is taken with the inspection apparatus, a microstructure of a template that is periodically repeated as in a memory cell region is not resolved and is observed to have substantially uniform luminance of a gray level between a white level and a black level. If such a microstructure that is periodically repeated has a defect, the periodicity of repetitive patterns in an image of the microstructure is disturbed and a luminance change occurs in the image of the gray level according to the degree of the defect. The inspection apparatus detects such a luminance change caused by disturbance in the periodicity based on comparison between images, thereby detecting a defect in the microstructure that is unresolvable. 
         [0007]    However, the luminance values of the gray level change according to the dimension of the repetitive patterns or the dimensional ratio thereof. Therefore, if the dimension or the dimensional ratio of the repetitive patterns varies within the plane of a template, the luminance values of the gray level also fluctuate correspondingly. In this case, the gray level serving as a reference differs between comparison images, which causes a problem that it is difficult to detect a defect. 
       SUMMARY 
       [0008]    An inspection method according to the embodiments includes applying light of a light source to an inspection target; receiving light from the inspection target to obtain a first image of the inspection target by a sensor; based on an image of a first pattern comprising repetitive patterns unresolvable with a wavelength of the light source in the first image, calculating a deviation of luminance values with respect to each of first regions in the first pattern by a processor; obtaining a second image of the inspection target by the sensor; correcting luminance values of the second image by the processor based on the deviations of the luminance values; and comparing the repetitive patterns of the corrected second image with each other by a comparer. 
         [0009]    The deviation of the luminance values of each of the first regions may be a difference between a luminance average value of each of the first regions in the first image and a luminance average value of the first pattern in the first image. 
         [0010]    The correcting luminance values of the second image may be subtracting deviations of luminance values of the first pattern from luminance values of the first pattern in the second image. 
         [0011]    The method may further include calculating correction values by inverting signs of deviations of luminance values in the first pattern, wherein correcting luminance values of the second image may be adding the correction values to luminance values of the first pattern in the second image. 
         [0012]    The first image may include the first pattern and a second pattern resolvable with the light of the light source, and deviations of luminance values in the first pattern may be calculated after extracting luminance values of the first pattern from the first image or eliminating the second pattern from the first image. 
         [0013]    The first and second patterns may include second regions arranged to be periodically repeated, and in comparing the repetitive patterns with each other, comparison of each of the first and second patterns may be performed with respect to each of the second regions, comparison of the first pattern may be performed using the second image corrected, and comparison of the second pattern may be performed using the second image uncorrected. 
         [0014]    The method may further include comparing between luminance of a first repetitive pattern and luminance of a second repetitive pattern in the second image corrected, and determining that the inspection target has a defect when a luminance difference between the first repetitive pattern and the second repetitive pattern or a differential value of the luminance difference is larger than a first threshold. 
         [0015]    A size of the first regions may be smaller than that of the repetitive patterns and larger than that of pixels of the sensor. 
         [0016]    The inspection target may be determined to have a defect, luminance values of the second image may not be corrected, and the repetitive patterns may not be compared when the deviations of the luminance values are larger than a second threshold. 
         [0017]    An inspection method according to the embodiments includes: applying light of a light source to an inspection target; receiving light from the inspection target to obtain a first image of the inspection target by a sensor; based on an image of a first pattern comprising repetitive patterns unresolvable with a wavelength of the light source in the first image, calculating a maximum value, a minimum value, and an average value of luminance values with respect to each of predetermined regions in the first pattern; obtaining a second image of the inspection target by the sensor; correcting luminance values of the second image by a processor based on the maximum value, the minimum value, and the average value of the luminance values with respect to each of the predetermined regions; and comparing the repetitive patterns of the corrected second image with each other by a comparer. 
         [0018]    The processor may correct luminance values of the second image of the inspection target to substantially match maximum values of luminance values of the repetitive patterns compared by the comparer, substantially match minimum values of the luminance values of the repetitive patterns, and substantially match average values of the luminance values of the repetitive patterns. 
         [0019]    The first image of the inspection target may include the first pattern and a second pattern resolvable with the light of the light source, and the processor may calculate the maximum value, the minimum value, and the average value of the luminance values with respect to each of the predetermined regions after extracting luminance values of the first pattern from the first image of the inspection target or eliminating the second pattern from the first image of the inspection target. 
         [0020]    A pixel size of the first image may be larger than that of the second image. 
         [0021]    An inspection apparatus according to the embodiments include an optical system applying light to an inspection target; a sensor receiving light from the inspection target to obtain an image of the inspection target; a processor calculating, based on an image of a first pattern comprising repetitive patterns unresolvable with a wavelength of a light source of the optical system in the image of the inspection target, a deviation of luminance values with respect to each of first regions in the first pattern, the processor correcting luminance values of the image of the inspection target based on the deviations of the luminance values; and a comparer comparing the repetitive patterns of the corrected image with each other. 
         [0022]    The deviation of the luminance values of each of the first regions may be a difference between a luminance average value calculated for each of the first regions and a luminance average value of the first pattern in the image of the inspection target. 
         [0023]    The correcting luminance values of the image of the inspection target may be subtracting deviations of luminance values of the first pattern from luminance values of the first pattern in the image of the inspection target, or adding correction values obtained by inverting signs of the deviations of the luminance values of the first pattern to the luminance values of the first pattern in the image of the inspection target. 
         [0024]    The apparatus may further include a storage storing therein deviations of luminance values of the first pattern or correction values obtained by inverting signs of the deviations of the luminance values of the first pattern along with positional information. 
         [0025]    The image of the inspection target may include the first pattern and a second pattern resolvable with the light of the light source, and the processor may calculate deviations of luminance values in the first pattern after extracting luminance values of the first pattern from the image of the inspection target or eliminating the second pattern from the image of the inspection target. 
         [0026]    The processor and the comparer may not correct the luminance values of the image of the inspection target, may determine that the inspection target has a defect, and may not compare the repetitive patterns when the deviations of the luminance values are larger than a second threshold. 
         [0027]    An inspection apparatus includes an optical system applying light to an inspection target; a sensor receiving light from the inspection target to obtain an image of the inspection target; a processor calculating, based on an image of a first pattern comprising repetitive patterns unresolvable with a wavelength of a light source of the optical system in the image of the inspection target, a maximum value, a minimum value, and an average value of luminance values in the first pattern, the processor correcting luminance values of the image of the inspection target based on the maximum value, the minimum value, and the average value of the luminance values in the first pattern; and a comparer comparing the repetitive patterns of the corrected image with each other. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]      FIG. 1  illustrates an example of a configuration of an inspection apparatus according to a first embodiment; 
           [0029]      FIG. 2  is a schematic diagram illustrating an example of an in-plane luminance distribution of the template TMP; 
           [0030]      FIG. 3  is a flowchart illustrating an example of the operation of the inspection apparatus  1  according to the first embodiment; 
           [0031]      FIG. 4  illustrates an example of stripes being units for imaging; 
           [0032]      FIG. 5  is a conceptual diagram illustrating an example of a mesh and frames; 
           [0033]      FIGS. 6A and 6B  illustrate gray patterns of parts in the die D 1  and the die D 4  as units for comparison in the luminance deviation map; 
           [0034]      FIGS. 7A and 7B  are graphs of the luminance values corresponding to the gray patterns in  FIGS. 6A and 6B , respectively; 
           [0035]      FIGS. 8A and 8B  are graphs illustrating the luminance values of corrected gray patterns of the frames F 1  and F 2 , respectively; 
           [0036]      FIG. 9  is a plan view illustrating a part of a template TMP on which a resolving pattern and a non-resolving pattern are mixed; 
           [0037]      FIG. 10  is a flowchart illustrating an example of an operation of an inspection apparatus according to a second embodiment; and 
           [0038]      FIG. 11  is a flowchart illustrating an example of an operation of an inspection apparatus according to a third embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0039]    Embodiments will now be explained with reference to the accompanying drawings. The present invention is not limited to the embodiments. 
       First Embodiment 
       [0040]      FIG. 1  illustrates an example of a configuration of an inspection apparatus according to a first embodiment. An inspection apparatus  1  is an apparatus that optically images a template TMP to be used in the NIL technology and detects a defect on the template TMP. The inspection apparatus  1  can be applied to an inspection of a mask in an EUV (Extreme Ultraviolet) lithography technology. 
         [0041]    The inspection apparatus  1  includes a stage  10 , an optical system  20 , an image capturing system  30 , an autoloader  40 , a laser length-measurement system  50 , a control computer  100 , a luminance computing circuit  110 , an autoloader control circuit  120 , a stage control circuit  130 , a storage  140 , a display  150 , a comparison circuit  160 , a position circuit  170 , and motors M θ , M x , and M y . The luminance computing circuit  110  and the comparison circuit  160  can be configured as one computing circuit  101 . The stage  10  can have a template TMP as an inspection target placed thereon and can be relatively moved with respect to the optical system  20  by the motors M θ , M x , and M y . For example, the motor M θ  moves the stage  10  in a rotation direction (a θ direction) within a substantially horizontal plane. The motors M x  and M y  move the stage  10  in an X direction and a Y direction, respectively, within the substantially horizontal plane. The motors M θ , M x , and M y  moving the stage  10  enable the template TMP on the stage  10  to be scanned with light from the optical system  20 . 
         [0042]    The optical system  20  includes a light source  21 , a polarizing beam splitter  22 , a half-wave plate  23 , and an objective lens  25 . The light source  21  generates light to be applied to the template TMP. The polarizing beam splitter  22  reflects the light from the light source  21  to the template TMP and transmits reflection light reflected from the template TMP to the image capturing system  30 . The half-wave plate  23  provides a phase difference to a polarization plane of light from the template TMP. Light transmitted through the half-wave plate  23  is focused on the template TMP to irradiate the template TMP. The light reflected on the template TMP passes through the objective lens  25 , the half-wave plate  23 , and the polarizing beam splitter  22  to be received by the image capturing system  30 . The inspection apparatus  1  is a reflective inspection apparatus that receives the reflection light from the template TMP with the image capturing system  30  to obtain an optical image. However, the inspection apparatus  1  can be a transmissive inspection apparatus that receives light transmitted through the template TMP with the image capturing system  30  to obtain an optical image. 
         [0043]    The image capturing system  30  includes an image sensor  31  and a sensor circuit  32  and receives the light from the template TMP to obtain an image of the template TMP. The image sensor  31  receives the light from the optical system  20  and converts (photoelectric converts) the optical signal into an electrical signal. The image sensor  31  can be, for example, a line sensor including imaging elements such as photodiodes arranged in a line, or an area sensor including imaging elements arranged two-dimensionally in a plane. For example, the image sensor  31  can be a CCD (Charge Coupled Device). The sensor circuit  32  performs A/D (Analog-Digital) conversion of the electrical signal from the image sensor  31  to obtain an optical image. As explained later, this image is transmitted to the luminance computing circuit  110  and is used to obtain a deviation of luminance tone values (hereinafter, simply “luminances” or “luminance values”), or is transmitted to the comparison circuit  160  and is used in comparison processing at the time of detection of a defect on the template TMP. 
         [0044]    The autoloader  40  automatically transports the template TMP onto the stage  10  or automatically recovers the template TMP on the stage  10  according to a command from the autoloader control circuit  120 . 
         [0045]    The laser length-measurement system  50  detects positions of the stage  10  in the X direction and the Y direction and transmits positional information of the stage  10  to the position circuit  170 . 
         [0046]    The control computer  100  executes various types of control related to a defect inspection of the template TMP. The control computer  100  is connected to the luminance computing circuit  110 , the autoloader control circuit  120 , the stage control circuit  130 , the storage  140 , the display  150 , the comparison circuit  160 , and the position circuit  170  via a bus  105 . The storage  140  has information necessary for a defect inspection of the template TMP, defect data obtained by the defect inspection, and the like stored therein. The display  150  displays a defect image of the template TMP, coordinate data, and the like. 
         [0047]    The autoloader control circuit  120  controls the autoloader  40  to transport the template TMP in the manner described above. The stage control circuit  130  controls the motors M θ , M x , and M y  to appropriately operate the stage  10 . 
         [0048]    The position circuit  170  detects the position of the stage  10  in corporation with the laser length-measurement system  50 . The positional information of the stage  10  detected by the position circuit  170  is fed back to the stage control circuit  130 . The stage control circuit  130  controls the motors M θ , M x , and M y  to correctly move the stage  10 . The positional information of the stage  10  is transmitted also to the comparison circuit  160 . The comparison circuit  160  associates the positional information of the stage  10  with defect data obtained by comparison processing and stores the associated data in the storage  140 . This enables the display  150  to display the position of the defect data of the template TMP. 
         [0049]    The luminance computing circuit  110  receives the image of the template TMP from the sensor circuit  32  and calculates deviations of luminance values of an image of repetitive patterns that are too minute to be resolved with the wavelength of the light source  21 . The luminance computing circuit  110  corrects an image of the template TMP taken again based on the deviations of the luminance values. The luminance computing circuit  110  transmits the corrected image to the comparison circuit  160  when the comparison circuit  160  performs comparison of the image of the template TMP. 
         [0050]    The comparison circuit  160  receives the corrected image of the template TMP and performs comparison of the repetitive patterns by a die-die comparison method using the corrected image. In order to realize the functions explained above, the luminance computing circuit  110  and the comparison circuit  160  can be constituted of logic circuits or can be constituted of a CPU and programs. The luminance computing circuit  110  and the comparison circuit  160  can be combined as one computing circuit  101 . Some of the functions of the luminance computing circuit  110  can be incorporated into the comparison circuit  160  or some of the functions of the comparison circuit  160  can be incorporated into the luminance computing circuit  110 . More detailed functions of the luminance computing circuit  110  and the comparison circuit  160  will be explained later. 
         [0051]    The template TMP to be inspected is explained below. The template TMP used in the NIL technology has a mesa structure protruding from the surface of a glass substrate. A circuit pattern is formed on the mesa structure and the surface of the mesa structure is pushed onto a resist formed on a wafer to transfer the circuit pattern onto the resist. Therefore, the circuit pattern on the template TMP needs to be formed in a dimension of the same magnification as that of the dimension of the circuit pattern to be transferred onto the resist. For example, the line width and the space width of a line and space pattern are each formed in about a dozen nanometers to about several tens of nanometers, and the engraving depth of the spaces (the distance between the surface of the line pattern and the surface of the space pattern) is formed in about several tens of nanometers to about 100 nanometers. 
         [0052]    When such a template TMP is to be inspected, the inspection apparatus uses light of a wavelength of about 200 nanometers, which is close to the light source wavelength of a stepper, for example. However, as described above, if the circuit pattern on the template TMP is minuter than the wavelength of the light source of the inspection apparatus  1 , the inspection apparatus  1  cannot resolve the circuit pattern. The minimum diameter of a pattern that is resolvable with light of a certain wavelength is generally known as a Rayleigh resolution limit. When the resolution is R, the Rayleigh resolution limit is represented by Expression 1. 
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         [0053]    In this expression, λ is the wavelength of the light of the light source  21 , NA is the number of apertures, and k 1  is the coefficient depending on a focusing condition. NA is a value between about 0.7 to about 0.8 and k 1  is a value between about 0.5 and about 1. For example, when NA=0.7, k 1 =0.5, and λ=200 nanometers, the resolution limit dimension R is 143 nanometers. That is, if the line width or the space width becomes smaller than 143 nanometers, the light of the wavelength of 200 nanometer cannot provide a sufficient luminance amplitude corresponding to the pattern and cannot resolve the pattern. Such a minute pattern that is unresolvable with the light of the inspection apparatus  1  is hereinafter also referred to as “non-resolving pattern”. Meanwhile, a pattern that is resolvable with the light of the inspection apparatus  1  is hereinafter also referred to as “resolving pattern”. 
         [0054]    When a non-resolving pattern is a periodic (regular) repetitive pattern, an image taken by the image capturing system  30  has substantially uniform luminance values of a gray level between a white level and a black level and becomes a substantially flat gray pattern. The white level is, for example, a luminance level obtained when a flat portion (margin portion) including no pattern on the template TMP is imaged. The black level is, for example, a luminance level obtained in a state (light shielded state) in which the light from the light source  21  is shielded by a shutter (not illustrated) or the like. 
         [0055]    For example, a periodic repetitive pattern is used frequently for a memory cell region of a semiconductor memory device. The memory cell region often has a size downscaled to be smaller than the Rayleigh resolution limit. Therefore, a pattern on the template TMP used for formation of the memory cell region becomes a non-resolving pattern periodically repeated. When the image capturing system  30  images this non-resolving pattern of the template TMP, the obtained image becomes a gray pattern with a relatively-small luminance amplitude. 
         [0056]    When a periodical repetitive pattern is a line and space pattern, the luminance values of the gray pattern of the image change according to fluctuation in the pattern dimension or the dimensional ratio such as the line width, the space width, the duty ratio (the line width/the space width), and the aspect ratio (the engraving depth of the space pattern/the space width). For example, when the line width is enlarged and the duty ratio is increased, the luminance values of the gray pattern become closer to those of the white level. On the other hand, when the line width is narrowed and the duty ratio is decreased, the luminance values of the gray pattern become closer to those of the black level. 
         [0057]    When the periodical repetitive pattern is a hole pattern or a pillar pattern, the luminance values of the gray pattern of the image changes according to fluctuation in the dimension such as the hole diameter or the pillar diameter. 
         [0058]    Because same patterns are periodically arranged on these repetitive patterns, the image ideally has substantially uniform luminance values on the entire region of the repetitive patterns. However, the dimension and the dimensional ratio of the repetitive patterns actually vary within the plane of the template TMP in some cases. That is, the dimension and the dimensional ratio of the repetitive patterns have an in-plane distribution (in-plane deviations). In this case, the luminance values of the image also fluctuate according to the in-plane deviations of the dimension or the dimensional ratio. An example of the in-plane deviations of the luminance values of the image is illustrated in  FIG. 2 . 
         [0059]      FIG. 2  is a schematic diagram illustrating an example of an in-plane luminance distribution of the template TMP. This in-plane luminance distribution is a distribution of luminance values of an image obtained when a first pattern P 1  in the template TMP is imaged. The first pattern P 1  includes non-resolving patterns periodically repeated and is divided into six dies D 1  to D 6 . The dies D 1  to D 6  are units (second regions) periodically repeated and are formed to have the same pattern. Therefore, the dies D 1  to D 6  can correspond to semiconductor chips, respectively. However, the dies D 1  to D 6  are not limited thereto and can be arbitrary repetitive patterns. The dies D 1  to D 6  become units for comparison when compared by the comparison circuit  160 . 
         [0060]    In the example illustrated in  FIG. 2 , the luminance values are lower (closer to those of the black level) in the dies D 1  and D 3  and the luminance values are higher (closer to those of the white level) in the dies D 4  and D 6 . Therefore, it is estimated that, for example, the duty ratio (the line width/the space width) of the line and space pattern in the first pattern P 1  is relatively low on the sides of the die D 1  and the die D 3  and is relatively high on the sides of the die D 4  and the die D 6 . 
         [0061]    In this way, the dimension or the dimensional ratio of the repetitive pattern formed actually on the template TMP varies within the first pattern P 1  and accordingly the luminance values of the image of the first pattern P 1  also fluctuate within the plane. That is, the luminance values of the image being the gray pattern in the first pattern P 1  has in-plane deviations. 
         [0062]    If the luminance values of the gray pattern have in-plane deviations, the gray level serving as a reference when dies are compared by the die-die comparison method differs between comparison images. Therefore, it is difficult to detect a defect by simple image comparison. 
         [0063]    Accordingly, before the dies are compared by the comparison circuit  160 , the inspection apparatus  1  according to the first embodiment images the first pattern P 1  of the template TMP, obtains an in-plane luminance distribution from the luminance values of the image of the first pattern P 1 , and calculates a deviation of the luminance values with respect to each frame F (first region) based on the in-plane luminance distribution to obtain a luminance deviation map of the first pattern P 1 . Further, the inspection apparatus  1  images the first pattern P 1  of the template TMP again and corrects the image of the first pattern P 1  imaged again based on the deviations of the luminance values in the luminance deviation map. The dies are compared using the image corrected in this manner. 
         [0064]    An operation of the inspection apparatus  1  is explained in more detail below. 
         [0065]      FIG. 3  is a flowchart illustrating an example of the operation of the inspection apparatus  1  according to the first embodiment. 
         [0066]    First, a template TMP is placed on the stage  10  and plate rotation alignment of the template TMP is performed (Step S 10 ). In a typical case, alignment marks located in horizontal and vertical positional relations are provided on the template TMP at positions not affecting the operations of circuits, such as four corners of an outer-circumferential scribe line region. Plate alignment is an operation of aligning an X coordinate axis and a Y coordinate axis of an inspection target pattern on a transfer surface of the template TMP with a parallel direction and a perpendicular direction of a traveling axis of the stage  10 , respectively, using the alignment marks. This normalizes a rotation or expansion/contraction error of the inspection target pattern on the template TMP with respect to the optical system  20  of the inspection apparatus  1 . 
         [0067]    Optimization of the light intensity amplitude (dynamic range) of the image sensor  31  is also performed. For example, the dynamic range between the black level obtained when imaging is performed in a light shielded state and the white level obtained when a flat surface of the template TMP including no patterns is imaged is adjusted. 
         [0068]    Next, the template TMP is imaged to obtain an in-plane luminance distribution of the first pattern P 1  (Step S 20 ).  FIG. 4  illustrates an example of stripes being units for imaging. The template TMP is conceptually divided into stripes St 1  to St 4  being the units for imaging. The stripes St 1  to St 4  include a plurality of repetitive patterns of the first pattern P 1  and/or a second pattern P 2 . The first pattern P 1  is explained below and the second pattern P 2  will be explained later in “Mixed pattern including resolving pattern and non-resolving pattern”. The image capturing system  30  obtains an image of each of the stripes (St 1  to St 4 ) while moving the template TMP. For example, while continuously moving the stage  10  in the X direction, the image capturing system  30  scans the stripe St 1  to obtain an optical image of the stripe St 1 . Next, the stage  10  is moved in the Y direction to move the image capturing system  30  to the stripe St 2 . While continuously moving the stage  10  in the reverse direction of the X direction, the image capturing system  30  scans the stripe St 2  to obtain an optical image of the stripe St 2 . In this way, the image capturing system  30  scans the stripes St 1  to St 4  to obtain an image (first image) of the entire template TMP. The in-plane luminance distribution illustrated in  FIG. 2  is obtained in this manner. 
         [0069]    Subsequently, the luminance computing circuit  110  calculates a luminance value of each frame F (first region) in  FIG. 5  to create a luminance deviation map (Step S 30 ). The luminance deviation map is a map indicating a deviation of the luminance value for each frame F in the region of the first pattern P 1 . At this time, the luminance computing circuit  110  extracts the luminance values of the first pattern P 1  from the image and then calculates the average value of luminances of respective pixels in the frame F, or eliminates the second pattern P 2  from the image and then calculates the average value of luminances of respective pixels in the frame F. A deviation between the average of all luminance values in the first pattern P 1  and the luminance value of each frame F is calculated with respect to each frame F. The deviation of the luminance value of each frame F is stored in the storage  140  together with the position coordinates. In this way, the luminance deviation map is stored in the storage  140 . When a correction value obtained by inverting the sign of the deviation of the luminance value is used, a correction value map is stored in the storage  140 . 
         [0070]    The calculation of the deviation of the luminance value is explained in more detail. 
         [0071]      FIG. 5  is a conceptual diagram illustrating an example of a mesh and frames. As illustrated in  FIG. 5 , the luminance computing circuit  110  virtually partitions the gray pattern with a mesh M and calculates the average value of luminances with respect to each of squares (frames F) of the mesh M. While the size of the frames F as the first regions can be arbitrarily set, the size is at least larger than the pixel size of the image sensor  31  and is equal to or smaller than the dies D 1  to D 6 . For example, the size of the frames F can be set according to the pixel size of the image sensor  31 . When the pixel size is, for example, 50 nanometers per pixel, the length of one side of the frames F can be, for example, 25 micrometers corresponding to 500 pixels. Meanwhile, the length of one side of the respective dies D 1  to D 6  is, for example, several millimeters. In this way, the size of the mesh M is normally considerably finer than the die size. In  FIG. 5 , the mesh M is illustrated in an expediential manner for easy understanding and the scale thereof may be different from an actual one. It is assumed that the size of the frames F is sufficiently larger than the size (several nanometers to several tens of nanometers, for example) of defects to be detected. 
         [0072]    The luminance computing circuit  110  further calculates a difference between the luminance average value of each frame F and a reference value and regards the difference as a deviation of the luminance value in the first pattern P 1 . The reference value can be, for example, the average value of luminances in the entire region of the first pattern P 1 . It suffices to store the reference value in the storage  140  in advance. The deviation of the luminance value of each frame F is stored in the storage  140  along with the position coordinates of the relevant frame F. 
         [0073]    The luminance computing circuit  110  similarly calculates the luminance average value and the deviation of each of all the frames F in the region of the first pattern P 1 . In this way, the deviations of the luminance values of all the frames F in the region of the first pattern P 1  are obtained. 
         [0074]    The deviations of the luminance values of the respective frames F in the entire region of the first pattern p 1  are stored in the storage  140  as the luminance deviation map. For example, the deviation of the luminance value of the frame F 1  in the die D 1  is low and has a negative value, and the deviation of the luminance value of the frame F 2  in the die D 4  is high and has a positive value. Similarly, the deviation of the luminance value of the frame F 3  in the die D 3  is low and has a negative value, and the deviation of the luminance value of the frame F 4  in the die D 6  is high and has a positive value. The luminance deviation map obtained in this manner is used for image correction, which will be explained later. The luminance deviation map can be displayed on the display  150  as needed. When the frames F are finer than the dies D 1  to D 6 , the luminance deviation map is similar to the in-plane luminance distribution illustrated in  FIG. 2 . Therefore, illustrations of the entire luminance deviation map are omitted here. 
         [0075]    For example,  FIGS. 6A and 6B  illustrate gray patterns of parts (the frames F 1  and F 2  illustrated in  FIG. 5 , for example) in the die D 1  and the die D 4  as units for comparison in the luminance deviation map. Comparison target parts illustrated in  FIGS. 6A and 6B  are the frames F 1  and F 2  corresponding in the dies D 1  and D 4 , respectively.  FIGS. 7A and 7B  are graphs of the luminance values corresponding to the gray patterns in  FIGS. 6A and 6B , respectively.  FIG. 7A  illustrates the luminance values at positions on a line  7   a - 7   a  in  FIG. 6A , where the horizontal axis represents the distance from one end Ea of the line  7   a - 7   a.    FIG. 7B  illustrates the luminance values at positions on a line  7   b - 7   b  in  FIG. 6B , where the horizontal axis represents the distance from one end Eb of the line  7   b - 7   b.  The vertical axes in  FIGS. 7A and 7B  both represent the luminance value. 
         [0076]    As is apparent from  FIGS. 6A to 7B , the luminance values of the frame F 2  of the die D 4  are higher than those of the frame F 1  of the die D 1  and are closer to those of the white level. 
         [0077]    For example, it is assumed that a defect DEF is located on a center portion of the frame F 2  as illustrated in  FIG. 7B . In this case, the defect DEF appears as an isolated black point in the gray pattern as illustrated in  FIG. 6B . Therefore, as illustrated in  FIG. 7B , the luminance value of the defect DEF is lower than those of other regions. 
         [0078]    However, because the magnitude of the defect DEF to be detected is considerably smaller than the size of the frame F, the influence thereof on the luminance average value of the frame F 2  is small. Therefore, the deviation of the luminance value in the region of the first pattern P 1  calculated by the luminance computing circuit  110  results from an in-plane deviation of the dimension or the dimensional ratio (the duty ratio, for example) of the repetitive pattern in the region. Accordingly, the luminance deviation map represents the in-plane deviation of the dimension or the dimensional ratio of the repetitive pattern in the region of the first pattern P 1 . In order to eliminate the influence of the in-plane deviation of the dimension or the dimensional ratio of the repetitive pattern in the defect inspection, the luminance computing circuit  110  corrects the image as at Step S 50 , which will be explained later. 
         [0079]    Referring back to  FIG. 3 , next, the template TMP is imaged again to perform a comparative inspection (Step S 40 ). At this time, the image capturing system  30  takes an image (a second image) of the same region in the template TMP similarly at Step S 20 . The image capturing system  30  transmits the image of the template TMP imaged again to the luminance computing circuit  110 . 
         [0080]    Next, the luminance computing circuit  110  subtracts the deviation based on the luminance deviation map from the luminance value of each of pixels in the frame F (Step S 50 ). This smooths the deviations of the luminance values of the respective frames F. The luminance computing circuit  110  then transmits the corrected gray pattern of the first pattern P 1  to the comparison circuit  160 . 
         [0081]    The luminance computing circuit  110  is explained in more detail below. 
         [0082]    The luminance computing circuit  110  receives the image of the first pattern P 1  which has been imaged again by the image capturing system  30 , from the image capturing system  30 . The luminance computing circuit  110  subtracts the deviation of the luminance value in the luminance deviation map from the luminance value of each pixel in the first pattern P 1  with respect to each of the frames F. Such a correction of the luminance value of each pixel in the first pattern P 1  smooths the gray pattern of the first pattern P 1 . When the corrected gray pattern is displayed on the display  150 , the corrected gray pattern represents more uniform (flatter) luminance values than in the gray pattern before corrected. 
         [0083]    For example,  FIGS. 8A and 8B  are graphs illustrating the luminance values of corrected gray patterns of the frames F 1  and F 2 , respectively. The luminance values of the corrected gray patterns of the frames F 1  and F 2  are equal in the average value. Therefore, as illustrated in  FIGS. 8A and 8B , the deviations of the luminance values of the respective frames F are smoothed in regions other than the defect DEF. 
         [0084]    In the example described above, the luminance computing circuit  110  creates the luminance deviation map and subtracts the deviation of the luminance value in the luminance deviation map from the luminance value of each pixel in the first pattern P 1 . However, the luminance computing circuit  110  alternatively can calculate correction values by inverting the signs (positive or negative) of the deviations of the luminance values with respect to each of the frames F to create a correction value map. In this case, the correction value map is a complementary map in which the signs of the deviations of the luminance values of the respective frames F of the luminance deviation map are inverted. In this case, it suffices that the luminance computing circuit  110  adds the correction value based on the correction value map to the luminance value of each pixel in the image of the first pattern P 1  imaged again. In this way, also by using the correction value map complementary with the luminance deviation map instead of the luminance deviation map, the deviations of the luminance values of the respective frames F can be smoothed. 
         [0085]    Referring back to  FIG. 3 , next, the comparison circuit  160  performs die-die comparison processing using the corrected gray pattern of the first pattern P 1  (Step S 60 ). The comparison processing can be, for example, a level-difference comparison system or a differential comparison system, which will be explained later. The comparison circuit  160  compares two dies as comparison targets. At this time, because the first pattern P 1  has been already corrected, the dies can be compared in a state including the first pattern P 1  and the second pattern P 2  mixed. 
         [0086]    When the dies are compared and the absolute value of a luminance difference or the absolute value of a differential value of the luminance difference is larger than a threshold (YES at Step S 70 ), the comparison circuit  160  or the control computer  100  determines that the relevant frame has a defect (Step S 80 ). On the other hand, when the dies are compared and the absolute value of a luminance difference or the absolute value of a differential value of the luminance difference is equal to or smaller than the threshold (NO at Step S 70 ), the comparison circuit  160  or the control computer  100  determines that the relevant frame includes no defect (Step S 90 ). The determination result is stored in the storage  140  along with the position coordinates. 
         [0087]    The comparison circuit  160  is explained in more detail below. 
         [0088]    The comparison circuit  160  performs comparison processing on the image of the first pattern P 1  corrected in the manner as described above and detects a defect. The comparison processing is, for example, the die-die comparison method and the level-difference comparison system or the differential comparison system can be used. 
         [0089]    In the level-different comparison system, the comparison circuit  160  eliminates a difference between a luminance level (luminance average) of the entire die D 1  and the luminance level (luminance average) of the entire die D 4  using the corrected gray pattern. Further, when the absolute value of the luminance difference between the luminance of the die D 1  and the luminance of the die D 4  is larger than a threshold (first threshold), the comparison circuit  160  estimates that there is a defect at the relevant position of the template TMP. The luminance difference of the defect part and the coordinates thereof are stored in the storage  140 . It is sufficient to store the threshold in the storage  140  in advance. 
         [0090]    In the differential comparison system, after eliminating the difference between the luminance level of the entire die D 1  and the luminance level of the entire die D 4 , the comparison circuit  160  differentiates a luminance difference between the luminance of the die D 1  and the luminance of the die D 4 . Because there is no luminance difference between the die D 1  and the die D 4  at a position including no defect, the differential value is small. On the other hand, the luminance difference greatly changes between the die D 1  and the D 4  at a position including a defect and thus the differential value is large in the absolute value. Therefore, when the absolute value of the differential value is larger than a threshold (first threshold), the comparison circuit  160  estimates that there is a defect at the relevant position of the template TMP. The luminance difference (or the differential value) of the defect part and the coordinates thereof are stored in the storage  140 . It is sufficient to store the threshold in the storage  140  in advance. 
         [0091]    Either the level-different comparison system or the differential comparison system or both thereof can be performed. The comparison circuit  160  can detect a defect using other comparison systems. While the die D 1  and the die D 4  are compared in the example described above, other dies are compared similarly. The comparison method is not particularly limited. For example, the die D 1  can be used as a reference to compare each of the dies D 2  to D 6  with the die D 1 . Alternatively, adjacent dies being comparison targets can be changed in turn in such a manner that the dies D 1  and D 2  are compared with each other, the dies D 2  and D 3  are compared with each other, . . . . 
         [0092]    When the comparison processing for the two dies as the comparison targets ends, one or both of the two dies are changed and the processes at Steps S 60  to S 90  are performed again (NO at Step S 110 ). When the comparison processing for all the dies in the template TMP ends (YES at Step S 110 ), the inspection of the template TMP ends. 
         [0093]    Further, the inspection result is displayed as a defect map on the display  150  (Step S 130 ). For example, it suffices that the display  150  displays the inspection result stored in the storage  140  according to the coordinates. This enables the display  150  to display the position of the defect on the template TMP. 
         [0094]    As described above, the inspection apparatus  1  according to the first embodiment images the first pattern P 1  of the template TMP and obtains the in-plane luminance distribution from the luminance values of the image of the first pattern P 1 . The inspection apparatus  1  averages the luminance values with respect to each of the frames F to create the luminance deviation map (or the correction value map) of the first pattern P 1 . The inspection apparatus  1  images again the first pattern P 1  of the template TMP and corrects the image of the first pattern P 1  imaged again using the luminance deviation map (or the correction value map). With this correction, the luminance average values of the gray pattern become equal in the dies D 1  to D 6  as comparison targets. Meanwhile, because the size of a defect to be detected is sufficiently smaller than the size of the frames F, the defect does not affect the luminance deviation map (or the correction value map) so much even when the luminance values are averaged with respect to each of the frames F. Therefore, when correcting the image of the first pattern P 1  using the luminance deviation map (or the correction value map), the inspection apparatus  1  can smooth the in-plane deviations of the luminance values of the first pattern P 1  while keeping fluctuation in the luminance value due to the defect. Accordingly, the inspection apparatus  1  can easily detect a defect on a non-resolving pattern periodically repeated. 
         [0095]    For example, when the luminance average value of the gray pattern of the die D 1  and the luminance average value of the gray pattern of the die D 4  are to be matched, it is conceivable that a moving average is performed to smooth the luminance difference within a plane of the first pattern P 1  or that a weighted average of adjacent plural pixels is performed. However, if the moving average or the weighted average is performed, the luminance of a defect part is also averaged and attenuated. In this case, detection of a defect may become difficult. 
         [0096]    In contract thereto, the inspection apparatus  1  according to the first embodiment averages the luminance values in each of the frames F sufficiently larger than a defect, creates the luminance deviation map (or the correction value map), and corrects the luminance values of the image of the first pattern P 1  using the luminance deviation map (or the correction value map). Therefore, in the first embodiment, there is no need to perform averaging processing such as the moving average or the weighted average. Accordingly, the inspection apparatus  1  according to the first embodiment can suppress attenuation in the luminance of a defect part and thus can easily detect a defect on the template TMP. 
         [0000]    (Mixed Pattern including Resolving Pattern and Non-Resolving Pattern) 
         [0097]      FIG. 9  is a plan view illustrating a part of a template TMP on which a resolving pattern and a non-resolving pattern are mixed. On a semiconductor chip, a minute structure such as a memory cell region and a structure in which the line width is wide, such as a power source, a sense amplifier, and a driver, are mixed in some cases. In these cases, the template TMP has a mixed pattern including both a non-resolving pattern P 1  and a resolving pattern P 2  as illustrated in  FIG. 9 . The non-resolving pattern P 1  as the first pattern is a pattern that is unresolvable with the light of the light source of the optical system  20  and the resolving pattern P 2  as the second pattern is a pattern that is resolvable with the light of the light source of the optical system  20 . 
         [0098]    The mixed pattern is also sometimes a repetitive pattern that is periodically repeated. In this case, comparison is performed regarding a unit of repetition of the mixed pattern as a die to inspect both the first pattern P 1  and the second pattern P 2 . 
         [0099]    When the mixed pattern is imaged by the image capturing system  30 , the image of the non-resolving pattern P 1  has luminance values of the gray level and the image of the resolving pattern P 2  has an amplitude of the white and black levels. Therefore, if the resolving pattern P 2  is corrected similarly to the non-resolving pattern P 1 , the luminance is decreased or increased, which prevents the die-die comparison from being performed for the resolving pattern P 2 . 
         [0100]    Therefore, the correction using deviations of the luminance values according to the first embodiment is applied to the image corresponding to the non-resolving pattern P 1  and is not applied to the image of the resolving pattern P 2 . Accordingly, the luminance computing circuit  110  extracts the luminance values of the image of the non-resolving pattern P 1  from the image of the mixed pattern and then calculates the deviations of the luminance values in the non-resolving pattern P 1 . Alternatively, the luminance computing circuit  110  eliminates the image of the resolving pattern P 2  from the image of the mixed pattern and then calculates the deviations of the luminance values in the remaining non-resolving pattern P 1 . The calculation method of the deviations of the luminance values can be the same as that described above. 
         [0101]    For example, when the resolving pattern P 2  is located at outer edge portions of the respective dies D 1  to D 6  illustrated in  FIG. 2 , the luminance computing circuit  110  extracts the luminance values of the non-resolving pattern P 1  located at central portions of the dies D 1  to D 6  or eliminates the resolving pattern P 2  located at the outer edge portions of the dies D 1  to D 6  and then calculates the deviations of the luminance values in the non-resolving pattern P 1 . 
         [0102]    The deviations of the luminance values in the non-resolving pattern P 1  are used for correction of the non-resolving pattern P 1  in an image taken again when the die-die comparison is performed. The correction of the non-resolving pattern P 1  is identical to the correction of the image described above. At this time, the second pattern P 2  is not corrected. 
         [0103]    After the image of the non-resolving pattern P 1  is corrected, the comparison circuit  160  performs the die-die comparison of the image. At this time, the comparison is performed in a state including the non-resolving pattern P 1  and the resolving pattern mixed. Because the deviations of the luminance values in the non-resolving pattern P 1  are already corrected, the non-resolving pattern P 1  can be compared along with the resolving pattern P 2 . The comparison of the image can be performed by either the level-difference comparison system or the differential comparison system described above. 
         [0104]    In this way, also for a mixed pattern including the non-resolving pattern P 1  and the resolving pattern P 2 , the inspection apparatus  1  according to the first embodiment can correct only the image of the non-resolving pattern P 1  to enable the die-die comparison. 
       Second Embodiment 
       [0105]      FIG. 10  is a flowchart illustrating an example of an operation of an inspection apparatus according to a second embodiment. The configuration of the inspection apparatus according to the second embodiment can be identical to the configuration of the inspection apparatus according to the first embodiment. 
         [0106]    The inspection apparatus  1  according to the second embodiment stops the inspection without performing the correction processing for the image of the first pattern P 1  and the comparison processing for dies when the deviations of the luminance values in the first pattern P 1  are larger than a threshold (second threshold). Large deviations of the luminance values in the first pattern P 1  indicate that the in-plane deviations of the dimension or the dimensional ratio of the first pattern P 1  are large. Therefore, when the deviations of the luminance values are considerably large, it can be determined that the template TMP has in-plane deviations exceeding an acceptable value. The inspection apparatus  1  according to the second embodiment then stops the inspection processing when the deviations of the luminance values are larger than the threshold. 
         [0107]    For example, after processes at Steps S 10  and S 20 , the luminance computing circuit  110  calculates deviations of the luminance values in the first pattern P 1  and compares the deviations of the luminance values with the threshold (S 22 ). 
         [0108]    When a deviation of the luminance value of any of frames F in the first pattern P 1  is larger than the threshold in the absolute value (YES at Step S 22 ), the luminance computing circuit  110  and the comparison circuit  160  end the processing without performing the processes at Step S 30  to S 130 . In this case, the display  150  displays that the inspection is stopped and also displays the deviation of the luminance value together with the position of the corresponding frame F (Step S 24 ). This enables an operator to know the stop of the inspection and to easily recognize the position where the in-plane deviation is large. 
         [0109]    On the other hand, when the deviations of the luminance values of all the frames F in the first pattern P 1  are equal to or smaller than the threshold in the absolute value (NO at Step S 22 ), the luminance computing circuit  110  and the comparison circuit  160  perform the processes at Step S 30  to S 130  similarly in the first embodiment. 
         [0110]    In this way, the inspection apparatus  1  according to the second embodiment stops the inspection processing when the deviations of the luminance values in the first pattern P 1  are larger than the threshold. This can omit an unnecessary inspection and can reduce the inspection time when the template TMP has a large abnormality. 
       Third Embodiment 
       [0111]      FIG. 11  is a flowchart illustrating an example of an operation of an inspection apparatus according to a third embodiment. The configuration of the inspection apparatus according to the third embodiment can be identical to the configuration of the inspection apparatus according to the first embodiment. 
         [0112]    According to the third embodiment, the luminance computing circuit  110  obtains the maximum value, the minimum value, and the average value of the luminance values in the first pattern P 1  and corrects the image of the first pattern P 1  so as to substantially match the maximum values, the minimum values, and the average values of the luminance values between dies to be compared by the comparison circuit  160 . This correction is performed instead of the process at Step S 50  (correction using the deviations of the luminance values) in the first and second embodiments. 
         [0113]    For example, after performing the processes at Steps S 10  and S 20 , the luminance computing circuit  110  obtains the maximum value, the minimum value, and the average value of the luminance values in the first pattern P 1 . Maximum, minimum, and average value maps are created in this manner (Step S 32 ). In the case of a mixed pattern, the luminance computing circuit  110  extracts the luminance values of the first pattern P 1  from the image and then calculates the maximum, minimum, and average values of luminance values of each die, or eliminates the second pattern P 2  from the image and then calculates the maximum, minimum, and average values of the luminance values. The maximum, minimum, and average values of the luminance values are calculated with respect to each die and are calculated for all the dies in the first pattern P 1 . The maximum, minimum, and average values of the luminance values of each of the dies are stored in the storage  140  along with the position coordinates. The maximum, minimum, and average value maps are thus stored in the storage  140 . 
         [0114]    Next, after performing the process at Step S 40 , the luminance computing circuit  110  substantially matches the average values of the luminance values of two dies being the comparison targets in an image of the first pattern P 1  imaged again. Furthermore, the luminance computing circuit  110  also substantially matches the respective maximum values and minimum values thereof (Step S 44 ). For example, the luminance computing circuit  110  shifts the luminance values of the entire region of one of the two dies to cause the respective average values of the luminance values of the dies to coincide with each other. Next, the luminance computing circuit  110  calculates the gain (multiplication ratio) of the luminance values to cause the amplitudes of the maximum and minimum values of the luminance values of the two dies to coincide with each other. The luminance computing circuit  110  then multiplies the amplitude from the shifted average value of the luminance values of the correction target die by the calculated gain. In this way, the average values, the maximum values, and the minimum values of the luminance values of the two dies are substantially matched. Alternatively, the luminance computing circuit  110  shifts the luminance values of entire region of one of the two dies to cause the respective average values of the luminance values of the dies to coincide with each other. Next, the luminance computing circuit  110  calculates standard deviations of the luminance values of the two dies and calculates the gain (the magnification ratio) of the luminance values to cause the calculated standard deviations to coincide with each other. The luminance computing circuit  110  then multiplies the amplitude from the shifted average value of the luminance values of the correction target die by the calculated gain. This processing also can substantially match the average values, the maximum values, and the minimum values of the luminance values of the two dies. Thereafter, the inspection apparatus performs the processes at Step S 60  and subsequent steps similarly in the first embodiment. 
         [0115]    According to the third embodiment, dynamic ranges of two dies being comparison targets become substantially equal. Furthermore, the luminance average values of the two dies being the comparison targets also become equal. Accordingly, the gray levels of the dies are matched to some extent and thus a defect on the template TMP can be easily detected similarly in the first embodiment. 
         [0116]    Furthermore, according to the third embodiment, the maximum value, the minimum value, and the average value of the luminance values are calculated with respect to each die without requiring to calculate the deviation of the luminance values with respect to each frame F. Therefore, the maximum, minimum, and average value maps of the third embodiment can be created in a short time and the inspection time can be reduced as compared to that in the first and second embodiments. 
         [0117]    The third embodiment can be combined with the second embodiment. In this case, it suffices to perform the processes at Steps S 22  and S 24  in  FIG. 10  between the processes at Step S 20  and Step S 32  in  FIG. 11 . Due to this combination, the third embodiment can also achieve the effects of the second embodiment. 
       Fourth Embodiment 
       [0118]    The inspection apparatus  1  according to a fourth embodiment sets the pixel size of the image of the template TMP imaged at Step S 20  in  FIG. 3  larger (coarser) than the pixel size of the image of the template TMP imaged at Step S 40 . For example, the pixel size of the image of the template TMP is set to about 70 nanometers at Step S 20  and the pixel size of the image of the template TMP is set to about 50 nanometers at Step S 40 . 
         [0119]    In an image to be used to create the luminance deviation map (or the correction value map), the luminance values of each frame F are averaged to create the luminance deviation map (or the correction value map). Therefore, as long as the luminance values of each frame F can be averaged, there is no problem if the pixel size of the image of the template TMP imaged at Step S 20  is larger (coarser). Meanwhile, the creation time of the luminance deviation map (or the correction value map) can be reduced by setting the pixel size of the image larger (coarser) in this manner. 
         [0120]    The configuration and operations of the inspection apparatus  1  according to the fourth embodiment other than those described above can be the same as those in any of the first to third embodiment. Accordingly, the fourth embodiment can also achieve the effects of the first to third embodiments. 
         [0121]    While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel methods and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.