Patent Publication Number: US-2023145411-A1

Title: Pattern inspection apparatus, and method for acquiring alignment amount between outlines

Description:
CROSS-REFEREMCE TP RELATED APPLICATION 
     This application is a continuation application based upon and claims the benefit of priority from prior Japanese Patent Application No. 2020-128134 (application number) filed on Jul. 29, 2020 in Japan, and International Application PCT/JP2021/018380, the International Filing Date of which is May 14, 2021. The contents described in JP2020-128134 and PCT/JP2021/018380 are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     An embodiment of the present invention relates to an apparatus for inspecting patterns and a method for acquiring an alignment amount between outlines. For example, it relates to an inspection apparatus that performs inspection using a secondary electron image of a pattern emitted from the substrate irradiated with multiple electron beams, an inspection apparatus that performs inspection using an optical image of a pattern acquired from the substrate irradiated with ultraviolet rays, and a method for acquiring an alignment amount between outlines to be used for inspection. 
     Description of Related Art 
     In recent years, with advances in high integration and large capacity of the LSI (Large Scale Integrated circuits), the circuit line width required for semiconductor elements is becoming increasingly narrower. Because the LSI manufacturing requires an enormous production cost, it is essential to improve the yield. However, since patterns that make up the LSI have reached the order of 10 nanometers or less, dimensions to be detected as a pattern defect have become extremely small. Therefore, the pattern inspection apparatus for inspecting defects of ultrafine patterns exposed/transferred onto a semiconductor wafer needs to be highly accurate. Further, one of major factors that decrease the yield is due to pattern defects on the mask used for exposing/transferring ultrafine patterns onto a semiconductor wafer by the photolithography technology. Accordingly, the pattern inspection apparatus for inspecting defects on an exposure transfer mask used in manufacturing LSI needs to be highly accurate. 
     As a defect inspection method, there is known a method of comparing a measured image acquired by imaging a pattern formed on a substrate, such as a semiconductor wafer or a lithography mask, with design data or with another measured image acquired by imaging an identical pattern on the substrate. For example, as a pattern inspection method, there are “die-to-die inspection” and “die-to-database inspection”. The “die-to-die inspection” method compares data of measured images acquired by imaging identical patterns at different positions on the same substrate. The “die-to-database inspection” method generates, based on design data of a pattern, design image data (reference image), and compares it with a measured image being measured data acquired by imaging the pattern. Acquired images are transmitted as measured data to a comparison circuit. After performing an alignment between the images, the comparison circuit compares the measured data with reference data according to an appropriate algorithm, and determines that there is a pattern defect if the compared data do not match each other. 
     With respect to the pattern inspection apparatus described above, in addition to the apparatus that irradiates an inspection target substrate with laser beams in order to obtain a transmission image or a reflection image, there has been developed another inspection apparatus that acquires a pattern image by scanning an inspection target substrate with primary electron beams and detecting secondary electrons emitted from the inspection target substrate due to the irradiation with the primary electron beams. For such pattern inspection apparatus, it has been examined, instead of comparing pixel values, to extract an outline (contour line) of a pattern in an image, and use a positional relationship between the extracted outline and the outline of a reference image, as a determining index. For accurately comparing positions of outlines, it is necessary to perform an alignment with high precision between an outline of an inspection image and a reference outline. However, alignment processing between outlines is complicated compared with conventional alignment processing between images which minimizes a deviation in a luminance value of a pixel in each image by a least squares method, and thus, there is a problem that the processing takes a long time to perform a high-precision alignment. 
     The following method has been disclosed as a method for extracting an outline position on an outline, which is performed before alignment processing. In the disclosed method, edge candidates are obtained using a Sobel filter, etc., and then, a second differential value of a concentration value is calculated for each pixel of the edge candidates and adjacent pixels in the inspection region. Further, in two pixel groups adjacent to the edge candidates, one of the adjacent pixel groups which has more number of combinations of different signs of second differential values is selected as a pixel group of the second edge candidates. Then, using the second differential value of the edge candidate and that of the second edge candidate, edge coordinates of a detection target edge are obtained for each sub-pixel (e.g., refer to Japanese Patent Application Laid-open (JP-A) No. 2011-48592). 
     BRIEF SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, a pattern inspection apparatus includes 
     an image acquisition mechanism configured to acquire an inspection image of a substrate on which a figure pattern is formed; 
     an actual outline image generation circuit configured to generate an actual outline image of a predetermined region defined by a predetermined function, in which a gray scale value of each pixel in the predetermined region including a plurality of actual image outline positions on an actual image outline of the figure pattern in the inspection image is dependent on a distance from a center of a pixel concerned to a closest actual image outline position in the plurality of actual image outline positions; 
     a reference outline image generation circuit configured to generate a reference outline image of the predetermined region defined by the predetermined function, in which a gray scale value of each pixel in the predetermined region is dependent on a distance from a center of a pixel concerned to a closest reference outline position in a plurality of reference outline positions on a reference outline to be compared with the actual image outline; 
     an alignment amount calculation circuit configured to calculate an alignment amount for performing alignment between the actual outline image and the reference outline image by using a gray scale difference between the actual outline image and the reference outline image; and 
     a comparison circuit configured to compare the actual image outline with the reference outline by using the alignment amount. 
     According to another aspect of the present invention, a method for acquiring an alignment amount between outlines includes 
     acquiring an inspection image of a substrate on which a figure pattern is formed; 
     generating an actual outline image of a predetermined region defined by a predetermined function, in which a gray scale value of each pixel in the predetermined region including a plurality of actual image outline positions on an actual image outline of the figure pattern in the inspection image is dependent on a distance from a center of a pixel concerned to a closest actual image outline position in the plurality of actual image outline positions; 
     generating a reference outline image of the predetermined region defined by the predetermined function, in which a gray scale value of each pixel in the predetermined region is dependent on a distance from a center of a pixel concerned to a closest reference outline position in a plurality of reference outline positions on a reference outline to be compared with the actual image outline; and 
     calculating an alignment amount for performing alignment between the actual outline image and the reference outline image by using a gray scale difference between the actual outline image and the reference outline image, and outputting a result. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a diagram showing an example of a configuration of a pattern inspection apparatus according to a first embodiment; 
         FIG.  2    is a conceptual diagram showing a configuration of a shaping aperture array substrate according to the first embodiment; 
         FIG.  3    is an illustration of an example of a plurality of chip regions formed on a semiconductor substrate according to the first embodiment; 
         FIG.  4    is an illustration of a scanning operation with multiple beams according to the first embodiment; 
         FIG.  5    is a flowchart showing main steps of an inspection method according to the first embodiment; 
         FIG.  6    is a block diagram showing an example of a configuration in a comparison circuit according to the first embodiment; 
         FIG.  7    is a diagram showing an example of an actual image outline position according to the first embodiment; 
         FIG.  8    is a diagram for explaining an example of a method for extracting a reference outline position according to the first embodiment; 
         FIG.  9    is a diagram for explaining an example of an alignment amount according to a comparative example 1 of the first embodiment 1; 
         FIGS.  10 A and  10 B  are diagrams for explaining an outline image according to the first embodiment; 
         FIG.  11    is a table showing an example of an alignment calculation result between outline images according to the first embodiment; 
         FIG.  12    is an illustration for explaining a positional deviation amount in consideration of an alignment amount according to the first embodiment; 
         FIGS.  13 A and  13 B  are diagrams showing an example of an outline position estimated based on an actual outline image according to the first embodiment; 
         FIGS.  14 A and  14 B  are diagrams showing an example of an outline position estimated based on an actual outline image according to a comparative example 2 of the first embodiment; 
         FIG.  15    is a diagram showing an example of a shift amount error according to the comparative example 2 of the first embodiment; and 
         FIG.  16    is a diagram showing an example of a shift amount error according to the first embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention provide an apparatus and method that can acquire a highly precise alignment amount while suppressing the processing time. 
     First Embodiment 
     The embodiments below describe an electron beam inspection apparatus as an example of a pattern inspection apparatus. However, it is not limited thereto. For example, the inspection apparatus may be the one in which the inspection substrate, to be inspected, is irradiated with ultraviolet rays to obtain an inspection image using a light transmitted through the inspection substrate or reflected therefrom. Further, the embodiments below describe an inspection apparatus using multiple electron beams to acquire an image, but it is not limited thereto. The inspection apparatus using a single electron beam to acquire an image may also be employed. 
       FIG.  1    is a diagram showing an example of a configuration of a pattern inspection apparatus according to a first embodiment. In  FIG.  1   , an inspection apparatus  100  for inspecting a pattern formed on the substrate is an example of a multi-electron beam inspection apparatus. The inspection apparatus  100  includes an image acquisition mechanism  150  (secondary electron image acquisition mechanism) and a control system circuit  160 . The image acquisition mechanism  150  includes an electron beam column  102  (electron optical column) and an inspection chamber  103 . In the electron beam column  102 , there are disposed an electron gun  201 , an electromagnetic lens  202 , a shaping aperture array substrate  203 , an electromagnetic lens  205 , a collective blanking deflector  212 , a limiting aperture substrate  213 , an electromagnetic lens  206 , an electromagnetic lens  207  (objective lens), a main deflector  208 , a sub deflector  209 , an E×B separator  214  (beam separator), a deflector  218 , an electromagnetic lens  224 , an electromagnetic lens  226 , and a multi-detector  222 . In the case of  FIG.  1   , a primary electron optical system which irradiates a substrate  101  with multiple primary electron beams is composed of the electron gun  201 , the electromagnetic lens  202 , the shaping aperture array substrate  203 , the electromagnetic lens  205 , the collective blanking deflector  212 , the limiting aperture substrate  213 , the electromagnetic lens  206 , the electromagnetic lens  207  (objective lens), the main deflector  208 , and the sub deflector  209 . A secondary electron optical system which irradiates the multi-detector  222  with multiple secondary electron beams is composed of the E×B separator  214 , the deflector  218 , the electromagnetic lens  224 , and the electromagnetic lens  226 . 
     In the inspection chamber  103 , there is disposed a stage  105  movable at least in the x and y directions. The substrate  101  (target object) to be inspected is mounted on the stage  105 . The substrate  101  may be an exposure mask substrate, or a semiconductor substrate such as a silicon wafer. In the case of the substrate  101  being a semiconductor substrate, a plurality of chip patterns (wafer dies) are formed on the semiconductor substrate. In the case of the substrate  101  being an exposure mask substrate, a chip pattern is formed on the exposure mask substrate. The chip pattern is composed of a plurality of figure patterns. When the chip pattern formed on the exposure mask substrate is exposed/transferred onto the semiconductor substrate a plurality of times, a plurality of chip patterns (wafer dies) are formed on the semiconductor substrate. The case of the substrate  101  being a semiconductor substrate is mainly described below. The substrate  101  is placed, with its pattern-forming surface facing upward, on the stage  105 , for example. Further, on the stage  105 , there is disposed a mirror  216  which reflects a laser beam for measuring a laser length emitted from a laser length measuring system  122  arranged outside the inspection chamber  103 . The multi-detector  222  is connected, at the outside of the electron beam column  102 , to a detection circuit  106 . 
     In the control system circuit  160 , a control computer  110  which controls the whole of the inspection apparatus  100  is connected, through a bus  120 , to a position circuit  107 , a comparison circuit  108 , a reference outline position extraction circuit  112 , a stage control circuit  114 , a lens control circuit  124 , a blanking control circuit  126 , a deflection control circuit  128 , a storage device  109  such as a magnetic disk drive, a monitor  117 , and a memory  118 . The deflection control circuit  128  is connected to DAC (digital-to-analog conversion) amplifiers  144 ,  146  and  148 . The DAC amplifier  146  is connected to the main deflector  208 , and the DAC amplifier  144  is connected to the sub deflector  209 . The DAC amplifier  148  is connected to the deflector  218 . 
     The detection circuit  106  is connected to a chip pattern memory  123  which is connected to the comparison circuit  108 . The stage  105  is driven by a drive mechanism  142  under the control of the stage control circuit  114 . In the drive mechanism  142 , a drive system such as a three (x-, y-, and θ-) axis motor which provides drive in the directions of x, y, and θ in the stage coordinate system is configured, and therefore, the stage  105  can be moved in the x, y, and θ directions. A step motor, for example, can be used as each of these x, y, and θ motors (not shown). The stage  105  is movable in the horizontal direction and the rotation direction by the x-, y-, and θ-axis motors. The movement position of the stage  105  is measured by the laser length measuring system  122 , and supplied (transmitted) to the position circuit  107 . Based on the principle of laser interferometry, the laser length measuring system  122  measures the position of the stage  105  by receiving a reflected light from the mirror  216 . In the stage coordinate system, the x, y, and θ directions are set, for example, with respect to a plane perpendicular to the optical axis (center axis of electron trajectory) of the multiple primary electron beams. 
     The electromagnetic lenses  202 ,  205 ,  206 ,  207  (objective lens),  224  and  226 , and the E×B separator  214  are controlled by the lens control circuit  124 . The collective blanking deflector  212  is composed of two or more electrodes (or poles), and each electrode is controlled by the blanking control circuit  126  through a DAC amplifier (not shown). The sub deflector  209  is composed of four or more electrodes (or poles), and each electrode is controlled by the deflection control circuit  128  through the DAC amplifier  144 . The main deflector  208  is composed of four or more electrodes (or poles), and each electrode is controlled by the deflection control circuit  128  through the DAC amplifier  146 . The deflector  218  is composed of four or more electrodes (or poles), and each electrode is controlled by the deflection control circuit  128  through the DAC amplifier  148 . 
     To the electron gun  201 , there is connected a high voltage power supply circuit (not shown). The high voltage power supply circuit applies an acceleration voltage between a filament (cathode) and an extraction electrode (anode) (which are not shown) in the electron gun  201 . In addition to the applying the acceleration voltage, a voltage is applied to another extraction electrode (Wehnelt), and the cathode is heated to a predetermined temperature, and thereby, electrons from the cathode are accelerated to be emitted as an electron beam  200 . 
       FIG.  1    shows configuration elements necessary for describing the first embodiment. It should be understood that other configuration elements generally necessary for the inspection apparatus  100  may also be included therein. 
       FIG.  2    is a conceptual diagram showing a configuration of a shaping aperture array substrate according to the first embodiment. As shown in  FIG.  2   , holes (openings)  22  of m 1  columns wide (width in the x direction) (each column in the y direction) and n 1  rows long (length in the y direction) (each row in the x direction) are two-dimensionally formed at a predetermined arrangement pitch in the shaping aperture array substrate  203 , where one of m 1  and n 1  is an integer of 2 or more, and the other is an integer of 1 or more. In the case of  FIG.  2   , 23×23 holes (openings)  22  are formed. Ideally, each of the holes  22  is a rectangle (including a square) having the same dimension, shape, and size. Alternatively, ideally, each of the holes  22  may be a circle with the same outer diameter. m 1 ×n 1  (=N) multiple primary electron beams  20  are formed by letting portions of the electron beam  200  individually pass through a corresponding one of a plurality of holes  22 . 
     Next, operations of the image acquisition mechanism  150  in the inspection apparatus  100  will be described below. 
     The electron beam  200  emitted from the electron gun  201  (emission source) is refracted by the electromagnetic lens  202 , and illuminates the whole of the shaping aperture array substrate  203 . As shown in  FIG.  2   , a plurality of holes  22  (openings) are formed in the shaping aperture array substrate  203 . The region including all the plurality of holes  22  is irradiated by the electron beam  200 . The multiple primary electron beams  20  are formed by letting portions of the electron beam  200  applied to the positions of the plurality of holes  22  individually pass through a corresponding one of the plurality of holes  22  in the shaping aperture array substrate  203 . 
     The formed multiple primary electron beams  20  are individually refracted by the electromagnetic lenses  205  and  206 , and travel to the electromagnetic lens  207  (objective lens), while repeating forming an intermediate image and a crossover, passing through the E×B separator  214  disposed at the crossover position of each beam (at the intermediate image position of each beam) of the multiple primary electron beams  20 . Then, the electromagnetic lens  207  focuses the multiple primary electron beams  20  onto the substrate  101 . The multiple primary electron beams  20  having been focused on the substrate  101  (target object) by the objective lens  207  are collectively deflected by the main deflector  208  and the sub deflector  209  to irradiate respective beam irradiation positions on the substrate  101 . When all of the multiple primary electron beams  20  are collectively deflected by the collective blanking deflector  212 , they deviate from the hole in the center of the limiting aperture substrate  213  and are blocked by the limiting aperture substrate  213 . By contrast, the multiple primary electron beams  20  which were not deflected by the collective blanking deflector  212  pass through the hole in the center of the limiting aperture substrate  213  as shown in  FIG.  1   . Blanking control is provided by On/Off of the collective blanking deflector  212 , and thus On/Off of the multiple beams is collectively controlled. In this way, the limiting aperture substrate  213  blocks the multiple primary electron beams  20  which were deflected to be in the “Off condition” by the collective blanking deflector  212 . Then, the multiple primary electron beams  20  for inspection (for image acquisition) are formed by the beams having been made during a period from becoming “beam On” to becoming “beam Off” and having passed through the limiting aperture substrate  213 . 
     When desired positions on the substrate  101  are irradiated with the multiple primary electron beams  20 , a flux of secondary electrons (multiple secondary electron beams  300 ) including reflected electrons, each corresponding to each of the multiple primary electron beams  20 , is emitted from the substrate  101  due to the irradiation with the multiple primary electron beams  20 . 
     The multiple secondary electron beams  300  emitted from the substrate  101  travel to the E×B separator  214  through the electromagnetic lens  207 . 
     The E×B separator  214  includes a plurality of more than two magnetic poles of coils, and a plurality of more than two, electrodes (poles). For example, the E×B separator  214  includes four magnetic poles (electromagnetic deflection coils) whose phases are mutually shifted by 90°, and four electrodes (electrostatic deflection electrodes) whose phases are also mutually shifted by 90°. For example, by setting two opposing magnetic poles to be an N pole and an S pole, a directive magnetic field is generated by these plurality of magnetic poles. Also, for example, by applying electrical potentials V whose signs are opposite to each other to the two opposing electrodes, a directive electric field is generated by these plurality of electrodes. Specifically, the E×B separator  214  generates an electric field and a magnetic field to be orthogonal to each other in a plane perpendicular to the traveling direction of the center beam (i.e., electron trajectory center axis) of the multiple primary electron beams  20 . The electric field exerts a force in a fixed direction regardless of the traveling direction of electrons. In contrast, the magnetic field exerts a force according to Fleming&#39;s left-hand rule. Therefore, the direction of the force acting on (applied to) electrons can be changed depending on the entering (or “traveling”) direction of electrons. With respect to the multiple primary electron beams  20  entering the E×B separator  214  from above, since the forces due to the electric field and the magnetic field cancel each other out, the beams  20  travel straight downward. In contrast, with respect to the multiple secondary electron beams  300  entering the E×B separator  214  from below, since both the forces due to the electric field and the magnetic field are exerted in the same direction, the multiple secondary electron beams  300  are bent obliquely upward, and separated from the multiple primary electron beams  20 . 
     The multiple secondary electron beams  300  having been bent obliquely upward and separated from the multiple primary electron beams  20  are further bent by the deflector  218 , and projected onto the multi-detector  222  while being refracted by the electromagnetic lenses  224  and  226 . The multi-detector  222  detects the projected multiple secondary electron beams  300 . Reflected electrons and secondary electrons may be projected on the multi-detector  222 , or it is also acceptable that reflected electrons are diffused (emitted) along the way and remaining secondary electrons are projected. The multi-detector  222  includes a two-dimensional sensor. Then, each secondary electron of the multiple secondary electron beams  300  collides with its corresponding region of the two-dimensional sensor, thereby generating electrons, and secondary electron image data is generated for each pixel. In other words, in the multi-detector  222 , a detection sensor is disposed for each primary electron beam of the multiple primary electron beams  20 . Then, the detection sensor detects a corresponding secondary electron beam emitted by irradiation with each primary electron beam. Therefore, each of a plurality of detection sensors in the multi-detector  222  detects an intensity signal of a secondary electron beam for an image resulting from irradiation with an associated primary electron beam. The intensity signal detected by the multi-detector  222  is output to the detection circuit  106 . 
       FIG.  3    is an illustration of an example of a plurality of chip regions formed on a semiconductor substrate according to the first embodiment. In  FIG.  3   , in the case of the substrate  101  being a semiconductor substrate (wafer), a plurality of chips (wafer dies)  332  are formed in a two-dimensional array in an inspection region  330  of the semiconductor substrate (wafer). A mask pattern for one chip formed on an exposure mask substrate is reduced to, for example, ¼, and exposed/transferred onto each chip  332  by an exposure device, such as a stepper and a scanner, (not shown). The region of each chip  332  is divided, for example, in the y direction into a plurality of stripe regions  32  by a predetermined width. The scanning operation by the image acquisition mechanism  150  is carried out, for example, for each stripe region  32 . The operation of scanning the stripe region  32  advances relatively in the x direction while the stage  105  is moved in the −x direction, for example. Each stripe region  32  is divided in the longitudinal direction into a plurality of rectangular (including square) regions  33 . Beam application to a target rectangular region  33  is achieved by collectively deflecting all the multiple primary electron beams  20  by the main deflector  208 . 
       FIG.  4    is an illustration of a scanning operation with multiple beams according to the first embodiment.  FIG.  4    shows the case of multiple primary electron beams  20  of 5 rows×5 columns. The size of an irradiation region  34  which can be irradiated by one irradiation with the multiple primary electron beams  20  is defined by (the x-direction size obtained by multiplying the x-direction beam pitch of the multiple primary electron beams  20  on the substrate  101  by the number of x-direction beams)×(the y-direction size obtained by multiplying the y-direction beam pitch of the multiple primary electron beams  20  on the substrate  101  by the number of y-direction beams). Preferably, the width of each stripe region  32  is set to be the same as the size in the y direction of the irradiation region  34 , or to be the size reduced by the width of the scanning margin. In the case of  FIGS.  3  and  4   , the irradiation region  34  and the rectangular region  33  are of the same size. However, it is not limited thereto. The irradiation region  34  may be smaller than the rectangular region  33 , or larger than it. A sub-irradiation region  29 , which is surrounded by the x-direction beam pitch and the y-direction beam pitch and in which the beam concerned itself is located, is irradiated and scanned (scanning operation) with each beam of the multiple primary electron beams  20 . Each primary electron beam  10  of the multiple primary electron beams  20  is associated with any one of the sub-irradiation regions  29  which are different from each other. At the time of each shot, each primary electron beam  10  is applied to the same position in the associated sub-irradiation region  29 . The primary electron beam  10  is moved in the sub-irradiation region  29  by collective deflection of all the multiple primary electron beams  20  by the sub deflector  209 . By repeating this operation, the inside of one sub-irradiation region  29  is irradiated with one primary electron beam  10  in order. Then, when scanning of one sub-irradiation region  29  is completed, the irradiation position is moved to an adjacent rectangular region  33  in the same stripe region  32  by collectively deflecting all of the multiple primary electron beams  20  by the main deflector  208 . By repeating this operation, the inside of the stripe region  32  is irradiated in order. After completing scanning of one stripe region  32 , the irradiation position is moved to the next stripe region  32  by moving the stage  105  and/or by collectively deflecting all of the multiple primary electron beams  20  by the main deflector  208 . As described above, a secondary electron image of each sub-irradiation region  29  is acquired by irradiation with each primary electron beam  10 . By combining secondary electron images of respective sub-irradiation regions  29 , a secondary electron image of the rectangular region  33 , a secondary electron image of the stripe region  32 , or a secondary electron image of the chip  332  is configured. 
     As shown in  FIG.  4   , each sub-irradiation region  29  is divided into a plurality of rectangular frame regions  30 , and a secondary electron image (image to be inspected) in units of frame regions  30  is used for inspection. In the example of  FIG.  4   , one sub-irradiation region  29  is divided into four frame regions  30 , for example. However, the number used for the dividing is not limited to four, and other number may be used for the dividing. 
     It is also preferable to group, for example, a plurality of chips  332  aligned in the x direction in the same group, and to divide each group into a plurality of stripe regions  32  by a predetermined width in the y direction, for example. Then, moving between stripe regions  32  is not limited to the moving in each chip  332 , and it is also preferable to move in each group. 
     When the multiple primary electron beams  20  irradiate the substrate  101  while the stage  105  is continuously moving, the main deflector  208  executes a tracking operation by performing collective deflection so that the irradiation position of the multiple primary electron beams  20  may follow the movement of the stage  105 . Therefore, the emission position of the multiple secondary electron beams  300  changes every second with respect to the trajectory central axis of the multiple primary electron beams  20 . 
     Similarly, when the inside of the sub-irradiation region  29  is scanned, the emission position of each secondary electron beam changes every second in the sub-irradiation region  29 . Thus, the deflector  218  collectively deflects the multiple secondary electron beams  300  so that each secondary electron beam whose emission position has changed as described above may be applied to a corresponding detection region of the multi-detector  222 . 
       FIG.  5    is a flowchart showing main steps of an inspection method according to the first embodiment. In  FIG.  5   , the inspection method of the first embodiment executes a series of steps: a scanning step (S 102 ), a frame image generation step (S 104 ), an actual image outline position extraction step (S 106 ), a reference outline position extraction step (S 110 ), an actual outline image generation step (S 120 ), a reference outline image generation step (S 122 ), an alignment amount calculation step (S 130 ), a defective positional deviation amount calculation step (S 142 ), and a comparison step (S 144 ). 
     In the scanning step (S 102 ), the image acquisition mechanism  150  acquires an image of the substrate  101  on which a figure pattern has been formed. Specifically, the image acquisition mechanism  150  irradiates the substrate  101 , on which a plurality of figure patterns has been formed, with the multiple primary electron beams  20  to acquire a secondary electron image of the substrate  101  by detecting the multiple secondary electron beams  300  emitted from the substrate  101  due to the irradiation with the multiple primary electron beams  20 . As described above, reflected electrons and secondary electrons may be projected on the multi-detector  222 , or alternatively, reflected electrons are diffused along the way, and only remaining secondary electrons (the multiple secondary electron beams  300 ) may be projected thereon. 
     As described above, the multiple secondary electron beams  300  emitted from the substrate  101  due to the irradiation with the multiple primary electron beams  20  are detected by the multi-detector  222 . Detected data (measured image data: secondary electron image data: inspection image data) on the secondary electron of each pixel in each sub irradiation region  29  detected by the multi-detector  222  is output to the detection circuit  106  in order of measurement. In the detection circuit  106 , the detected data in analog form is converted into digital data by an A-D converter (not shown), and stored in the chip pattern memory  123 . Then, acquired measured image data is transmitted to the comparison circuit  108 , together with information on each position from the position circuit  107 . 
       FIG.  6    is a block diagram showing an example of a configuration in a comparison circuit according to the first embodiment. In  FIG.  6   , in the comparison circuit  108  of the first embodiment, there are arranged storage devices  50 ,  52 ,  56 , and  57  such as magnetic disk drives, a frame image generation unit  54 , an actual image outline position extraction unit  58 , an actual image shortest distance calculation unit  60 , an actual outline image generation unit  62 , a reference shortest distance calculation unit  64 , a reference outline image generation unit  66 , an alignment amount calculation unit  68 , a defective positional deviation amount calculation unit  82  and a comparison processing unit  84 . Each of the “units” such as the frame image generation unit  54 , the actual image outline position extraction unit  58 , the actual image shortest distance calculation unit  60 , the actual outline image generation unit  62 , the reference shortest distance calculation unit  64 , the reference outline image generation unit  66 , the alignment amount calculation unit  68 , the defective positional deviation amount calculation unit  82  and the comparison processing unit  84  includes processing circuitry. The processing circuitry includes, for example, an electric circuit, computer, processor, circuit board, quantum circuit, semiconductor device, or the like. Further, common processing circuitry (the same processing circuitry), or different processing circuitry (separate processing circuitry) may be used for each of the “units”. Input data required in the frame image generation unit  54 , the actual image outline position extraction unit  58 , the actual image shortest distance calculation unit  60 , the actual outline image generation unit  62 , the reference shortest distance calculation unit  64 , the reference outline image generation unit  66 , the alignment amount calculation unit  68 , the defective positional deviation amount calculation unit  82 , and the comparison processing unit  84 , and calculated results are stored in a memory (not shown) or in the memory  118  each time. 
     The measured image data (scan image) transmitted into the comparison circuit  108  is stored in the storage device  50 . 
     In the frame image generation step (S 104 ), the frame image generation unit  54  generates a frame image  31  of each of a plurality of frame regions  30  obtained by further dividing the image data of the sub-irradiation region  29  acquired by a scanning operation with each primary electron beam  10 . In order to prevent missing an image, it is preferable that margin regions overlap each other in respective frame regions  30 . The generated frame image  31  is stored in the storage device  56 . 
     In the actual image outline position extraction step (S 106 ), the actual image outline position extraction unit  58  extracts, for each frame image  31 , a plurality of outline positions (actual image outline positions) of each figure pattern in the frame image  31  concerned. 
       FIG.  7    is a diagram showing an example of an actual image outline position according to the first embodiment. The method for extracting an outline position may be the conventional one. For example, differential filter processing for differentiating each pixel in the x and y directions by using a differentiation filter, such as a Sobel filter is performed to combine x-direction and y-direction primary differential values. Then, the peak position of a profile using the combined primary differential values is extracted as an outline position on an outline (actual image outline).  FIG.  7    shows the case where one outline position is extracted for each of a plurality of outline pixels through which an actual image outline passes. The outline position is extracted per sub-pixel in each outline pixel. In the example of  FIG.  7   , the outline position is represented by coordinates (x, y) in a pixel. Further, shown is a normal direction angle θ at each outline position of the outline approximated by fitting a plurality of outline positions by a predetermined function. The normal direction angle θ is defined by a clockwise angle to the x axis. Information on each obtained actual image outline position (actual image outline data) is stored in the storage device  57 . 
     In the reference outline position extraction step (S 110 ), reference outline position extraction circuit  112  extracts a plurality of reference outline positions for comparing with a plurality of actual image outline positions. A reference outline position may be extracted from design data. Alternatively, first, a reference image is generated from design data, and a reference outline position may be extracted using the reference image by the same method as that of the case of the frame image  31  being a measured image. Alternatively, a plurality of reference outline positions may be extracted by the other conventional method. 
       FIG.  8    is a diagram for explaining an example of a method for extracting a reference outline position according to the first embodiment. The case of  FIG.  8    shows an example of a method for extracting a reference outline position from design data. In  FIG.  8   , the reference outline position extraction circuit  112  reads design pattern data (design data) being a basis of a pattern formed on the substrate  101  from the storage device  109 . The reference outline position extraction circuit  112  sets grids, each being the size of a pixel, for the design data. The midpoint of a straight line in a quadrangle corresponding to a pixel is defined as a reference outline position. If there is a corner of a figure pattern, the corner vertex is defined as a reference outline position. If there are a plurality of corners, the intermediate point of the corner vertices is defined as a reference outline position. By the process described above, the outline position of a figure pattern as a design pattern in the frame region  30  can be extracted with sufficient accuracy. Information (reference outline data) on each obtained reference outline position is output to the comparison circuit  108 . Then, in the comparison circuit  108 , reference outline data is stored in the storage device  52 . 
       FIG.  9    is a diagram for explaining an example of an alignment amount according to a comparative example 1 of the first embodiment 1. FIG.  9    shows a portion of an outline (actual image outline) in the frame image  31  used as an image to be inspected, and a portion of a reference outline extracted from design data. The case of  FIG.  9    shows an actual image outline connecting a plurality of outline positions (actual image outline positions) extracted from the frame image  31 , and a reference outline connecting a plurality of outline positions (reference outline positions) extracted from design data. In the comparative example 1 of the first embodiment 1, a shift amount from an actual image outline position to the closest reference outline position is treated as an alignment amount. In the case of the comparative example 1, a true shift amount is not necessarily the same as a relative distance between outline positions. Therefore, an error occurs between a shift amount from an actual image outline position to the closest reference outline position (that is, a shift amount between closest outline positions) and a true shift amount. In particular, when a shift amount is large, an error appears notably large. In order to obtain a correct/accurate shift amount, it is necessary to estimate a highly accurate actual image outline instead of an actual image outline just connecting a plurality of actual image outline positions. The same applies to a reference outline. For estimating a highly accurate outline, a complicated calculation is needed, and therefore, the processing takes a long time. By contrast, if the accuracy of a shift amount deteriorates, false defects increase, and thus, it becomes difficult to perform an inspection with great accuracy. Then, according to the first embodiment, not directly performing alignment between outlines, but generating an outline image in which the gray scale value (pixel value) of each pixel is defined using a predetermined function, alignment is performed between outline images by using the pixel value. It is specifically described below. 
     In the actual outline image generation step (S 120 ), first, with respect to each pixel in the frame region  30  (predetermined region) including a plurality of actual image outline positions on an actual image outline of a figure pattern in the frame image  31  (inspection image), the actual image shortest distance calculation unit  60  calculates a distance L from the center of the pixel concerned to the closest actual image outline position in a plurality of actual image outline positions. 
       FIGS.  10 A and  10 B  are diagrams for explaining an outline image according to the first embodiment. In each example of  FIGS.  10 A and  10 B , a region of 3×3 pixels is shown. One outline position is extracted for each of a plurality of pixels through which an outline passes. In the case of  FIG.  10 A , one outline position is shown in the middle of each pixel row in the x direction being arrayed in the y direction. The actual image shortest distance calculation unit  60  calculates the distance L from the center of each of the 3×3 pixels to the closest outline position. Thereby, a distance L ij  can be acquired for each pixel. ij indicates coordinates (index) of a pixel. 
     Next, the actual outline image generation unit  62  generates an actual outline image of the frame region  30  defined by a predetermined function, whose derivative is substantially continuous and in which the gray scale value of each pixel in the frame region  30  including a plurality of actual image outline positions on an actual image outline of a figure pattern in the frame image  31  is dependent on the distance L ij  from the center of the pixel concerned to the closest actual image outline position in a plurality of actual image outline positions. According to the first embodiment, an actual outline image indicating outline positions is generated based on outline data. It is preferable that a Gaussian function is used as the predetermined function whose derivative is substantially continuous. The Gaussian function F(L) can be defined, as an example, by the following equation (1) using a distance L ij , an amplitude A, and a standard deviation σ. 
         F ( L   ij )= A ·exp (− L   ij   2 /2σ 2 )   (1)
 
     The amplitude A and the standard deviation σ can be set as appropriate. In the case of defining the gray scale value of each pixel of an outline image by unsigned 8 bits (0 to 255 gray scale levels), preferably, the amplitude A is set to be 255, for example. σ is a value indicating the spread of an outline image profile, and in general, the larger its numerical value, the smaller an error in a shift operation. However, if the minimum line width of a figure pattern or the minimum spacing between figure patterns becomes narrow, the bottom portion of the outline image profile and its adjacent outline overlap with each other, and therefore, an error occurs. Accordingly, σ is preferably set to be about ⅓ of the minimum line width or the minimum spacing of the figure pattern. 
     As shown in  FIG.  10 B , the value obtained by the Gaussian function F(L), for example, is defined as the gray scale value of each pixel. Thereby, the outline can be defined as a gray scale image. 
     In the reference outline image generation step (S 122 ), first, with respect to each pixel in a frame region in the frame image  31  (inspection image), the reference shortest distance calculation unit  64  calculates a distance L from the center of the pixel concerned to the closest reference outline position in a plurality of reference outline positions to be compared with an actual outline. The method for calculating the distance L is similar to the case of obtaining the distance to the actual image outline position shown in  FIG.  10 A . 
     Next, the reference outline image generation unit  66  generates a reference outline image of the frame region  30  defined by the above-described predetermined function shown in  FIG.  10 B , in which the gray scale value of each pixel in the frame region  30  (predetermined region) is dependent on the distance L from the center of the pixel concerned to the closest reference outline position in a plurality of reference outline positions on a reference outline to be compared with an actual image outline. 
     In the alignment amount calculation step (S 130 ), the alignment amount calculation unit  68  calculates an alignment amount for performing alignment between an actual outline image and a reference outline image by employing an evaluation function which uses a gray scale difference between the actual outline image and the reference outline image. Specifically, it operates as follows: 
       FIG.  11    is a table showing an example of an alignment calculation result between outline images according to the first embodiment. The alignment amount calculation unit  68  shifts at least one of an actual outline image and a reference outline image while changing the shift amount between the actual outline image and the reference outline image, and obtains the shift amount, as an alignment amount, that makes a sum of squared differences (SSD) between the gray scale value of the actual outline image and the gray scale value of the reference outline image in each shift amount smaller than others. Therefore,  FIG.  11    shows an example of an SSD calculation result at each position shifted per pixel in the x and y directions. In the case of  FIG.  11   , when the shift amount is x=1 pixel and y=1 pixel, the SSD calculation result is the minimum value. An outline image is shifted using the shift amount per pixel obtained described above, and then, by similarly performing shifting per sub pixel, an SSD is calculated at each position. The alignment amount calculation unit  68  calculates and obtains the shift amount that makes the sum of squared differences (SSD) per sub pixel smaller than others. Then, the shift amount per sub pixel is added to the shift amount per pixel described above to calculate a final alignment amount. 
     In the defective positional deviation amount calculation step (S 142 ), the defective positional deviation amount calculation unit  82  calculates a defective positional deviation amount in consideration of an alignment amount between each of a plurality of actual image outline positions and its closest reference outline position. 
       FIG.  12    is an illustration for explaining a positional deviation amount in consideration of an alignment amount according to the first embodiment. In order to accurately inspect whether a defect exists in an outline itself, it is necessary to perform an alignment with high precision between an actual image outline of the frame image  31  and a reference outline. According to the first embodiment, the defective positional deviation amount calculation unit  82  calculates a defective positional deviation amount (defective positional deviation vector (after alignment)) by subtracting an alignment amount (alignment vector) from a positional deviation amount (positional deviation vector (before alignment)) between an actual image outline position before alignment and a reference outline position. Thereby, the same effect as image alignment can be acquired. 
     In the comparison step (S 144 ), the comparison processing unit  84  (comparison unit) compares, using an alignment amount, an actual image outline with a reference outline. Specifically, the comparison processing unit  84  determines it as a defect when the magnitude (distance) of a defective positional deviation vector in consideration of an alignment amount between each of a plurality of actual image outline positions and its corresponding reference outline position exceeds a determination threshold. The comparison result is output to the storage device  109 , the monitor  117 , or the memory  118 . 
       FIGS.  13 A and  13 B  are diagrams showing an example of an outline position estimated based on an actual outline image according to the first embodiment.  FIG.  13 A  shows an example of an actual outline image of a portion of a figure pattern right side outline, where gray scale values of pixel values are represented by graded colors.  FIG.  13 B  shows an x-direction image profile. According to the first embodiment, since a value calculated based on a function (e.g. Gaussian function) whose derivative is approximately continuous is used for the gray scale value of each pixel of an actual outline image, it is possible to interpolate with a smooth curve when performing shift processing. Therefore, an error can be made small when an SSD value is calculated. Thus, in the alignment processing between an actual outline image and a reference outline image according to the first embodiment, it is possible to make the error small. 
       FIGS.  14 A and  14 B  are diagrams showing an example of an outline position estimated based on an actual outline image according to a comparative example 2 of the first embodiment.  FIG.  14 A  shows an actual outline image in which each pixel value is defined by a function whose derivative is discontinuous, such as a rectangular function not by a function (e.g. Gaussian function) whose derivative is continuous, where gray scale values of pixel values are represented by graded colors.  FIG.  14 B  shows an x-direction image profile. The rectangular function is defined by the following equation (2), for example. 
         F ( L )= A ·max (0, min (1 , S −abs ( L )))   (2)
 
     where, S: size of rectangle (S≥1, e.g., 3), A: amplitude (e.g., 255), min(x, y): the smaller one of x and y, max(x, y): the larger one of x and y, and abs(x): absolute value of x. 
     Next, in order to prevent an interpolation error of the rectangular function described above, averaging processing using 3×3 pixels is performed. Specifically, with respect to each of all the pixels of the outline image obtained by the rectangular function described above, the average value is calculated among nine pixels acquired by each pixel added by eight vertically, horizontally, and diagonally adjacent pixels. Then, the obtained average value is defined as a pixel value of the final outline image (equivalent to the conventional example image of  FIGS.  14 A and  14 B ). 
     Since, in the comparative example 2, a calculation result by a rectangular function, etc. whose derivative is discontinuous is used, it is difficult to interpolate with a smooth curve when performing shift processing. Therefore, the error of the SSD calculation becomes large. 
       FIG.  15    is a diagram showing an example of a shift amount error according to the comparative example 2 of the first embodiment.  FIG.  16    is a diagram showing an example of a shift amount error according to the first embodiment. In  FIGS.  15  and  16   , a one-dimensional image profile is generated to show a result of measuring an error between a true shift amount and a shift amount by an SSD. Here, positions of a reference image and an image to be shifted are shifted to change in units of sub pixels (0 to 1 pixel) to show a result of measuring errors with respect to all the combinations between them. In data series of  FIGS.  15  and  16   , the positions of the images to be shifted are based on the same conditions. 
     The abscissa axis represents the position (unit: pixel) of the reference image, and the ordinate axis represents an alignment error (unit: pixel). As shown in  FIG.  15   , in the comparative example 2, the maximum error is 0.0695 pixel. By contrast, as shown in  FIG.  16   , in the first embodiment, the maximum error is suppressed to 0.0074 pixel. According to the first embodiment, since the function (for example, Gaussian function) whose derivative is approximately continuous is used, the error of interpolation in the case of performing shift processing is small, and therefore, the alignment error can be made substantially small. 
     As described above, according to the first embodiment, the alignment amount between outlines can be calculated as an alignment amount between images using gray scale values of pixels obtained based on the function whose derivative is substantially continuous. Therefore, a highly precise deviation amount can be acquired while suppressing the processing time. 
     In the above description, a series of “. . . circuits” includes processing circuitry. The processing circuitry includes an electric circuit, computer, processor, circuit board, quantum circuit, semiconductor device, or the like. Each “. . . circuit” may use common processing circuitry (the same processing circuitry), or different processing circuitry (separate processing circuitry). A program for causing a processor, etc. to execute processing may be stored in a recording medium, such as a magnetic disk drive, flush memory, etc. For example, the position circuit  107 , the comparison circuit  108 , the reference outline position extraction circuit  112 , the stage control circuit  114 , the lens control circuit  124 , the blanking control circuit  126 , and the deflection control circuit  128  may be configured by at least one processing circuit described above. 
     Embodiments have been explained referring to specific examples described above. However, the present invention is not limited to these specific examples. Although  FIG.  1    shows the case where the multiple primary electron beams  20  are formed by the shaping aperture array substrate  203  irradiated with one beam from the electron gun  201  serving as an irradiation source, it is not limited thereto. The multiple primary electron beams  20  may be formed by irradiation with a primary electron beam from each of a plurality of irradiation sources. 
     While the apparatus configuration, control method, and the like not directly necessary for explaining the present invention are not described, some or all of them can be appropriately selected and used on a case-by-case basis when needed. 
     In addition, any pattern inspection apparatus, and method for acquiring an alignment amount between outlines that include elements of the present invention and that can be appropriately modified by those skilled in the art are included within the scope of the present invention. 
     Additional advantages and modification will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.