Abstract:
To address the problem in which when measuring the overlay of patterns formed on upper and lower layers of a semiconductor pattern by comparing a reference image and measurement image obtained through imaging by an SEM, the contrast of the SEM image of the pattern of the lower layer is low relative to that of the SEM image of the pattern of the upper layer and alignment state verification is difficult even if the reference image and measurement image are superposed on the basis of measurement results, the present invention determines the amount of positional displacement of patterns of an object of overlay measurement from a reference image and measurement image obtained through imaging by an SEM, carries out differential processing on the reference image and measurement image, aligns the reference image and measurement image that have been subjected to differential processing on the basis of the positional displacement amount determined previously, expresses the gradation values of the aligned differential reference image and differential measurement image as brightnesses of colors that differ for each image, superposes the images, and displays the superposed images along with the determined positional displacement amount.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a method of measuring misalignment (overlay) between patterns created in different manufacturing processes in manufacture of a semiconductor wafer, a device therefor, and a display method therefor, and more specifically to a method of measuring overlay by using an image obtained through photographing with a charged particle microscope, a device therefor, and a display device therefor. 
       BACKGROUND ART 
       [0002]    For semiconductor device products, a plurality of times of exposure processes are required to form a circuit pattern required for operation. For example, in manufacture of a device formed of multilayered circuit patterns, in addition to the exposure process for forming each layer of the circuit pattern, an exposure process for forming a hole connecting together the layers is required. Position of the circuit patterns formed through the aforementioned plurality of times of exposure processes needs to fall within a permitted range, and upon deviation from the permitted range, appropriate electric characteristics cannot be obtained, resulting in yield deterioration. Thus, measurement of circuit pattern misalignment (overlay) between the exposures and feedback thereof to an exposure device have been practiced. 
         [0003]    Following miniaturization of semiconductor processes, the permitted overlay range has become smaller and thus has become important to directly measure the overlay in a place where the product circuit pattern is formed. To realize this, Japanese Patent Application Laid-open No. 2013-168595 (Patent Literature 1) describes a technique of photographing an image of a product circuit pattern with a scanning electron microscope (SEM) and measuring overlay. 
         [0004]    The overlay measurement method described in Patent Literature 1 measures the overlay through image positional alignment between a reference image and a measured image, and  FIG. 30  in Patent Literature 1 describes a method of providing a display of measurement results as detection results of a differential part between the reference image and the measured image. 
         [0005]    Japanese Unexamined Patent Application Publication No. 2005-521254 describes a method of coloring a reference image and an inspected image to make a difference therebetween visible. More specifically, the method refers to an inspection method of image comparison between the reference image and the inspected image and a method of obtaining an inspected defective image by image coupling of a framework image of the colored reference image, an edge framework image of the colored reference image, and an inspected object of the colored inspected image. 
       CITATION LIST 
     Patent Literature 
       [0006]    PTL 1: Japanese Patent Application Laid-open No. 2013-168595 
         [0007]    PTL 2: Japanese Unexamined Patent Application Publication No. 2005-521254 
       SUMMARY 
     Technical Problem 
       [0008]    Patent Literature 1 describes that the overlay measurement based on the product circuit patterns of a semiconductor device is carried out by performing image positional alignment between the different patterns of the reference image and the measurement image targeted for the overlay measurement. Comparison between the reference image and the measurement image is a practical method in the overlay measurement targeted on the product circuit patterns, but it is required to visually check a state of the image positional alignment for the purpose of adjustment of an image processing parameter used for the image positional alignment and for the purpose of confirmation of the measurement results. 
         [0009]    In a case where a targeted process of the overlay measurement is a hole process, that is, in a case where there is positional misalignment between a hole formed at an upper layer and a pattern formed at a lower layer located at a hole bottom observed through the aforementioned hole, gray of an image obtained by photographing, with an SEM, the pattern formed at the lower layer observed through the hole bottom is relatively smaller than gray of an SEM image of the hole pattern formed at the upper layer, and thus upon direct mutual superposition of the reference image and the measurement image obtained in the hole process by photographing with a scanning electron microscope, it is difficult to visually confirm a state of the positional alignment of the hole bottom pattern. 
         [0010]    Patent Literature 1 describes that as the method of displaying the results of the positional alignment between the reference image and the measurement image, the detection results of the differential part between the reference image and the measurement image are displayed. Described as the method of detecting a differential part is a method of calculating a difference in a gray value after the positional alignment between the reference image and the measurement image and defining, as the differential part, a region including pixels where a value of the difference becomes equal to or larger than a given value. 
         [0011]      FIG. 1  and  FIG. 2  illustrates schematic diagrams of an image obtained by photographing, with an SEM, hole patterns formed on a circuit pattern of a semiconductor in a hole formation process. Illustrated in the schematic diagrams is how a lower layer pattern  103  at a hole bottom of a hole pattern  102  formed at a surface layer  101  is viewed.  FIG. 1  is the schematic diagram illustrating a state in which there is no misalignment (overlay) between the hole pattern  102  and the lower layer pattern (pad in examples of  FIG. 1  and  FIG. 2 )  103 , resulting in a reference image.  FIG. 2  illustrates a measurement image in a state in which there is misalignment (overlay) between the hole pattern  102  and the lower layer pattern  103  at the hole bottom. It is illustrated that an a base  201  of the layer on which the lower layer pattern  103  viewed at the hole bottom of the hole pattern  102  is viewed is observed darker. On the scanning electron microscope, electrons detected from the pattern formed at the lower layer at the hole bottom of the hole pattern  102  is smaller than electrons detected from the upper layer  101  on the surface, so that the lower layer pattern  103  at the hole bottom becomes darker. 
         [0012]      FIG. 3  and  FIG. 4  are obtained by mutually superposing and line-drawing the reference image of  FIG. 1  and the measurement image of  FIG. 2 . Broken lines of  FIG. 3  and  FIG. 4  represent an edge  304  or  404  of the hole pattern  102  and the lower layer pattern  103  at the hole bottom in the reference image of  FIG. 1 , and solid lines represent an edge  303  or  403  of the hole pattern  102  and an edge  302  or  402  of the lower layer pattern  103  at the hole bottom in the measurement image of  FIG. 2 . With the method described in Patent Literature  1 , image positional alignment is performed on an individual pattern basis. A diagrammatic view  301  of  FIG. 3  represents an example where the positional alignment is performed properly with the edge  304  of the lower layer pattern  103  at the hole bottom obtained from the measurement image of  FIG. 1  and the edge  302  of the lower layer pattern  103  at the hole bottom obtained from the measurement image of  FIG. 2 , and a diagrammatic view  401  of  FIG. 4  represents an example where the positional alignment was not performed properly with the edge  404  of the lower layer pattern  103  at the hole bottom obtained from the reference image of  FIG. 1  and the edge  402  of the lower layer pattern  103  at the hole bottom obtained from the measurement image of  FIG. 2 . 
         [0013]    Ways of superposition of regions  311  to  313  and  411  to  415  marked with numerals in  FIG. 3  and  FIG. 4  are different from that of regions  101  to  103  and  201  marked with numerals in the reference image of  FIG. 1  and the measurement image of  FIG. 2 , and there is also a difference in a gray value between the reference image of  FIG. 1  and the measurement image of  FIG. 2 . In actual images, there is brightness non-uniformity in the regions marked with the numerals in  FIG. 1  and  FIG. 2 , and since the regions marked with the numerals in  FIG. 1  and  FIG. 2  are photographed at different positions of the device, thus resulting in non-uniformity in the gray value. Thus, the gray value roughly differs among the regions illustrated in  FIG. 3  and  FIG. 4 , but the difference therebetween becomes more unclear, thus resulting in difficulties in clearly indicating the regions as illustrated in  FIG. 3  and  FIG. 4  in the difference in the gray value. Therefore, as in Patent Literature 1, even when a region such that a differential part between the reference image and the measurement image becomes equal to or larger than a given value is displayed, it is difficult to judge whether or not image positional alignment has properly been performed by each pattern. 
         [0014]    Patent Literature 2 describes a method of performing coloring on a reference image and an inspected image and detecting a difference between the reference image and the inspected image. However, positional alignment between the reference image and the inspected image is not performed with reference to either one of the patterns targeted for overlay measurement, thus resulting in failure to display results of the image positional alignment performed as the overlay measurement. Moreover, even when the positional alignment is performed with each pattern targeted for the overlay measurement, an image which permits judgement whether or not the image positioning has properly been performed cannot be obtained even by image coupling, through logical calculation, a frame image of the reference image or an edge image and an inspected object of the inspected image. 
         [0015]    Bold lines  501  and  601  of  FIG. 5  and  FIG. 6  indicate results of taking, as inspected objects, a logical product of the edge image  302  or  402  of the lower layer pattern  103  at the hole bottom of the reference image in  FIG. 3  and  FIG. 4  and regions  312 ,  412 , and  414  of the lower layer pattern of the measurement image in  FIG. 3  and  FIG. 4 . Thin lines  502  and  503  of  FIG. 5  and thin lines  602  to  604  of  FIG. 6  refer to line drawings illustrated in  FIG. 3  and  FIG. 4 . With only the bold lines  501  and  601  illustrated in  FIG. 5  and  FIG. 6 , it cannot be judged whether or not the positional alignment has properly been performed. 
         [0016]    The present invention address the problem of the conventional art, and provides, in a method of overlay measurement through comparison between a reference image and a measurement image based on a product circuit image of a semiconductor device obtained by photographing with a scanning electron microscope, an overlay measurement method, a device and a display device capable of easily confirming results of the comparison between the reference image and the measurement image. 
       Solution to Problem 
       [0017]    To address the problem described above, the present invention refers to a method of measuring overlay between patterns formed at different layers of a semiconductor device, and the method includes: acquiring a reference image including a pattern without overlay as misalignment between the pattern formed at the upper layer of the semiconductor device and the pattern formed at the lower layer thereof by using a scanning electron microscope; acquiring a measurement image including the pattern targeted for the measurement and formed at the upper layer of the semiconductor device and the pattern formed at the lower layer thereof by using the scanning electron microscope; calculating a positional misalignment amount of the patterns corresponding to the acquired reference image and the acquired measurement image; generating a differential reference image and a differential measurement image through differential processing performed on the acquired reference image and the acquired measurement image; generating a colored differential reference image through coloring with a first color having an intensity value corresponding to a gray value of the generated differential reference image and generating a colored differential measurement image through coloring with a second color being different from the first color and having an intensity value corresponding to a gray value of the generated differential measurement image; performing positional correction on the colored differential reference image or the colored differential measurement image by using information of the calculated positional misalignment amount of the pattern; and mutually superposing the colored differential reference image and the colored differential measurement image subjected to the positional correction and displaying the colored differential reference image and the colored differential measurement image together with the information of the calculated positional misalignment amount of the patterns. 
         [0018]    Moreover, to address the problem described above, the invention refers to an overlay measurement device which measures overlay of patterns formed at different layers of a semiconductor device, and the overlay measurement device includes: scanning electron microscopic adapted to acquire a reference image by imaging a region including the pattern without overlay as misalignment between the pattern formed at the upper layer of the semiconductor device and the pattern formed at the lower layer thereof, and to acquire a measurement image by imaging a region including the pattern targeted for the measurement and formed at the upper layer of the semiconductor device and the pattern targeted for the measurement and formed at the lower layer of the semiconductor device; positional misalignment amount calculator adapted to calculate an amount of positional misalignment between the patterns corresponding to the reference image and the measurement image acquired by the scanning electron microscope; differential image generator adapted to generate a differential reference image and a differential measurement image by subjecting, to differential processing, the reference image and the measurement image acquired by the scanning electron microscope; colored differential image generator adapted to generate a colored differential reference image by coloring in a first color having an intensity value corresponding to a gray value of the differential reference image generated by the differential image generator, and generating a colored differential measurement image by coloring in a second color being different from the first color and having an intensity value corresponding to a gray value of the differential measurement image generated by the differential image generator; image positional corrector adapted to perform positional correction on the colored differential reference image or the colored differential measurement image generated by the colored differential image generator by using information of the amount of the positional misalignment between the patterns calculated by the positional misalignment amount calculator; and display unit adapted to mutually superpose the colored differential reference image and the colored differential measurement image subjected to the positional correction performed by the image positional corrector, and to display the colored differential reference image and the colored differential measurement image together with the information of the amount of the positional misalignment between the patterns calculated by the positional misalignment amount calculator. 
         [0019]    Further, to address the problem described above, the invention refers to a device displaying measurement results of overlay of patterns formed at different layers of a semiconductor device, the measurement being achieved through comparison between a reference image in a region including the pattern without overlay as misalignment between the pattern formed at the upper layer of the semiconductor device and the pattern formed at the lower layer thereof both of which are acquired by photographing with a scanning electron microscope and a measurement image in a region including the pattern targeted for the measurement and formed at the upper layer of the semiconductor device and the pattern targeted for the measurement and formed at the lower layer of the semiconductor device, wherein a colored differential reference image obtained by coloring in a first color having an intensity value corresponding to a gray value of a differential filter image of the reference image and a colored differential measurement image obtained by coloring in a second color different from the first color and having an intensity value corresponding to a gray value of a differential filter image of the measurement image are superposed on each other to be displayed. 
       Advantageous Effects of Invention 
       [0020]    With one aspect of the present invention, in overlay measurement performed with a product circuit of a semiconductor device through comparison between a reference image and a measurement image by using an SEM image obtained by photographing the semiconductor device with a scanning electron microscope (SEM), when the reference image and the measurement image are shifted for superposed display thereof, the superposed display can be performed with favorable visibility. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0021]      FIG. 1  is a schematic diagram of an image obtained by photographing, with a scanning electron microscope, a semiconductor device on which a hole pattern has been formed through a hole pattern formation process, illustrating a state in which there is no misalignment between the hole pattern and a pattern formed at a lower layer. 
           [0022]      FIG. 2  is a schematic diagram of a measurement image in a hole process, i.e., the image obtained by photographing, with the scanning electron microscope, the semiconductor device on which the hole pattern has been formed through the hole pattern formation process, illustrating a state in which there is misalignment between the hole pattern and the pattern formed at the lower layer. 
           [0023]      FIG. 3  is a line drawing obtained by extracting edges of the patterns respectively observed on the reference image of  FIG. 1  and the measurement image of  FIG. 2 , illustrating a state in which positional alignment has been properly performed based on the pattern edges extracted from the respective images. 
           [0024]      FIG. 4  is a line drawing obtained by extracting the edges of the patterns respectively observed on the reference image of  FIG. 1  and the measurement image of  FIG. 2 , illustrating a state in which the positional alignment has not been properly performed based on the pattern edges extracted from the respective images. 
           [0025]      FIG. 5  is a diagrammatic view illustrating, in a bold line, an edge portion of a pattern formed at the lower layer of the hole pattern as a result of taking AND of an edge image of the pattern formed at the lower layer of the hole pattern in the reference image and a region of the pattern formed at the lower layer of the hole pattern in the measurement image, which are provided as inspected objects. 
           [0026]      FIG. 6  is a diagrammatic view illustrating, in a bold line, an edge portion of a pattern formed at the lower layer of the hole pattern as a result of taking AND of an edge image of the pattern formed at the lower layer of the hole pattern in the reference image of  FIG. 4  and a region of the pattern formed at the lower layer of the hole pattern in the measurement image, which are provided as inspected objects. 
           [0027]      FIG. 7  is a block diagram illustrating schematic configuration of an overlay measurement device according to Example of the present invention. 
           [0028]      FIG. 8  is a partially sectional view of a semiconductor device having a contact plug formed at a lower layer and a hole pattern formed on an upper layer, illustrating a state in which there is no misalignment between the contact plug at the lower layer and the hole pattern at the upper layer. 
           [0029]      FIG. 9  is a schematic diagram of an image obtained by photographing the semiconductor device in the state of  FIG. 8  with a scanning electron microscope. 
           [0030]      FIG. 10  is a partially sectional view of a semiconductor device having a contact plug formed a lower layer and a hole pattern formed at an upper layer, illustrating a state in which there is misalignment between the contact plug at the lower layer and the hole pattern at the upper layer. 
           [0031]      FIG. 11  is a schematic diagram of an image obtained by photographing the semiconductor device in the state of  FIG. 10  with the scanning electron microscope. 
           [0032]      FIG. 12  is a flowchart illustrating procedures of processing of calculating overlay according to Example of the invention. 
           [0033]      FIG. 13  is a flowchart illustrating procedures of generating an image displaying the overlay according to Example of the invention. 
           [0034]      FIG. 14  is a flowchart illustrating procedures of generating a reference image and a measurement image involved in mask processing according to a modified example of Example of the invention. 
           [0035]      FIG. 15  is a diagram illustrating a state in which display images  1310  and  1315  are superposed on each other according to the modified example of Example of the invention. 
           [0036]      FIG. 16  is a diagram displaying, in a vector, a change in a representative position of a hole pattern in the state in which the display images  1310  and  1315  are superposed on each other according to the modified example of Example of the invention. 
           [0037]      FIG. 17  illustrates an example of a display image in a state in which a measurement image is superposed on the display image  1310  of  FIG. 13  according to the modified example of Example of the invention. 
           [0038]      FIG. 18  illustrates an example of a display image in a state in which a reference image is superposed on the display image  1310  of  FIG. 13  according to the modified example of Example of the invention. 
           [0039]      FIG. 19  is a flowchart illustrating procedures of processing of measuring overlay according to Example of the invention. 
           [0040]      FIG. 20  is an elevation view of a display screen illustrating configuration of a screen displaying results of measuring the overlay according to Example of the invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0041]    The present invention relates to measurement of pattern overlay between layers circuit patterns formed at multiple layers of a semiconductor device, and permits superposed display with favorable visibility when a reference image and a measurement image provided by an SEM image obtained by imaging the circuit patterns are subjected to positional correction in accordance with an overlay amount obtained by using the reference image and the measurement image and are displayed in a mutually superposed manner. 
         [0042]    Hereinafter, embodiments of the present invention will be described with reference to the drawings. 
       Embodiment 1 
       [0043]      FIG. 7  shows an overall configuration diagram of an overlay measurement device  1000  according to the present invention. The overlay measurement device  1000  includes a scanning electron microscope device  700  and a processing and control section  750 . 
         [0044]    The scanning electron microscope device  700  includes: a stage  706  on which a semiconductor wafer  707  is loaded; an irradiation optical system  710  controlling an electron beam  701  emitted from an electron gun  702 ; and a detector  708  detecting a secondary electron or a reflective electron  709  emitted from above a sample (the semiconductor wafer  707 ) to which the electron beam  701  is irradiated. The irradiation optical system  710  includes the electron gun  702 , capacitor lenses  703 ; deflection coils  704 , and objective lenses  705  which are located on a route of the electron beam  701 . The electron beam  701  is focused by the irradiation optical system  710  on a predetermined region in which a defect to be observed is exist on the semiconductor wafer  707 . 
         [0045]    The processing and control section  750  includes: an A/D converter  751 , an image processing section  752 , an overall control section  753 , an electron optical system control section  754 , a stage controller  755 , and a display terminal  756 . 
         [0046]    On the scanning electron microscope device  700 , a detection signal outputted from the detector  708  that has detected the secondary electron or the reflective electron emitted from the semiconductor wafer  707 , to which the electron beam  701  has been emitted, is converted into a digital signal by the A/D converter  751 . The digital signal obtained through the conversion is transmitted to the image processing section  752 , in which image processing is performed using the digital signal transmitted from the A/D converter  751 , for example, detection of a defective position in an image is performed, and then results of the aforementioned operation is outputted to display at the display terminal  756  via the overall control section  753 . In response to a control signal from the overall control section  753 , the stage controller  755  drives the stage  706  of the scanning electron microscope device  700 . In response to the control single from the overall control section  753 , the electron optical system control section  754  controls, for example, the electron gun  702 , the capacitor lenses  703 , the deflection coils  704 , and the objective lenses  705  of the scanning electron microscope device  700 . 
         [0047]    A recording medium (not illustrated) can be connected to the image processing section  752 , the overall control section  753 , and the display terminal  756 , and a program to be executed in the image processing section  752  can be read from the recording medium and loaded onto the image processing section  752 . 
         [0048]    In a description of overlay measurement below, an example of a device pattern formed on the semiconductor wafer  707  and targeted for the measurement will be described with reference to schematic diagrams of  FIG. 8  and  FIG. 10  and an example of a captured image of the device pattern will be described with reference to  FIG. 9  and  FIG. 11 . 
         [0049]      FIG. 8  is a schematic cross-sectional view with a simplified partial cross section of a semiconductor device having a contact plug  804  formed at a lower layer  802  (although omitted from illustration, a thin film of a contact pad having almost the same diameter as that of the contact plug is formed on a surface of the contact plug  804 , and in the example of  FIG. 1 , the contact pad is actually observed) and having a hole pattern  803  formed at an upper layer  801 , illustrates a state in which the contact plug  804  at the lower layer and the hole pattern  803  at the upper layer are arrayed without misalignment. 
         [0050]      FIG. 9  illustrates an SEM image obtained upon imaging a region including the hole pattern  803  of  FIG. 8  from the above by using the scanning electron microscope device  700 , and the upper layer  801  of  FIG. 8  is imaged as a region  901 , the contact plug  804  at the lower layer is imaged as a region  902 , and an outline of the hole pattern  803  is imaged as a pattern  903 . Hereinafter, the contact plug  804  which is observed through the hole pattern  803  and formed at the lower layer  802  in the SEM image obtained by imaging with the scanning electron microscope device  700  is called the lower layer pattern  902 . The SEM image corresponding to the hole pattern  803  created at the upper layer  801  may simply be called the upper layer pattern  903 . 
         [0051]      FIG. 10  is a schematic cross-sectional view with a simplified partial cross section of a semiconductor device having a contact plug  1004  formed at a lower layer  1002  and a hole pattern  1003  formed at an upper layer  1001 , illustrating a state in which the contact plug  1004  at the lower layer and the hole pattern  1003  at the upper layer are being misaligned with each other. 
         [0052]      FIG. 11  illustrates an SEM image of a region  1101  corresponding to a surface of the upper layer  1001  obtained by imaging a region including the hole pattern  1003  of  FIG. 10  from the above by using the scanning electron microscope device  700 , and with respect to an outline  1103  of the hole pattern  1003 , a region  1102  of the contact plug  1004  at the lower layer is misaligned, and the lower layer  1002  is imaged as a region  1104  which is darkest in  FIG. 11 . 
         [0053]    The overlay measurement is intended to find misalignment between the hole pattern  803  ( 1003 ) formed at the upper layer  801  ( 1001 ) and the contact plug  804  ( 1004 ) formed at the lower layer  802  ( 1002 ). The captured images are processed to measure the misalignment between the lower layer pattern  902  ( 1002 ) in the SEM image illustrated in  FIG. 9  ( FIG. 11 ) corresponding to the contact plug  804  ( 1004 ) formed at the lower layer  802  ( 1002 ) in the sectional view of  FIG. 8  ( FIG. 10 ) and the hole pattern expressed by the upper layer pattern  903  ( 1103 ) in the SEM image illustrated in  FIG. 9  ( FIG. 11 ) corresponding to the hole pattern  803  ( 1003 ) formed at the upper layer  801  ( 1001 ) in the sectional view of  FIG. 8  ( FIG. 10 ). 
         [0054]    The pattern  804  or  1004  formed at the lower layer  802  or  1002  has been described as the contact plug in  FIG. 8  and  FIG. 10 , but the pattern formed at the lower layer  802  or  1002  is not limited to the contact plug, and a different pattern is also permitted. Moreover, the hole pattern  903  or  1103  is illustrated in a circle in  FIG. 9  and  FIG. 11 , but a shape of the hole in the image is not limited to the circle. 
         [0055]    In the overlay measurement performed through comparison between the reference image and the measurement image described in the present embodiment, an image in a state in which the two patterns targeted for the overlay measurement as in  FIG. 9 , that is, the upper layer pattern  903  corresponding to the hole pattern  803  and the lower layer pattern  902  corresponding to the contact plug  804  in the aforementioned example are aligned is provided as a reference image. The reference image may be created by imaging one or a plurality of places through user selection. In the overlay measurement performed through comparison between a reference image and a measurement image described below by referring  FIG. 12 , the region  902  and the outline  903  of the reference image are used as hole and outline information of the region on the surface of the upper layer, and the region  902  and the outline  903  are used as outline and region information of a hole bottom of the hole pattern  803  serving as a surface of boundary with the lower layer pattern. 
         [0056]      FIG. 12  shows a flow of overlay measurement processing performed through comparison between the reference image and the measurement image in the SEM image of the semiconductor wafer  707  obtained by using the scanning electron microscope device  700 . First, patterns targeted for the overlay measurement are extracted respectively from a reference image  1201  (corresponding to the image described in  FIG. 9 ) including a lower layer pattern image  12011  and a measurement image  1202  (corresponding to the image described in  FIG. 11 ) including a lower layer pattern image  12021  targeted for the overlay measurement and a lower layer image  12022  (S 1251 ) to create upper layer pattern images  1211  and lower layer pattern images  1212 . As the upper layer pattern images  1211 , a reference image  1203  including a hole pattern image  1204  and a measurement image  1205  including a hole pattern image  1206  are created. As the lower layer pattern images  1212 , a reference image  1207  including a contact plug pattern image  1208  corresponding to the lower layer pattern image  12011  and a measurement image  1209  including a contact plug pattern image  1210  corresponding to the lower layer pattern image  12021  are created. Extraction of the upper layer pattern images  1211  and the lower layer pattern images  1212  can be performed by a technique of region division in image processing, such as region division based on a gray level. 
         [0057]    Next, image positional alignment between the reference image  1203  and the measurement image  1205  is performed in the created upper layer pattern images  1211  (S 1252 ), and an amount (ΔUx, ΔUy) of the positional misalignment between the hole pattern image  1204  in the reference image  1204  included in the upper layer pattern images  1211  and the hole pattern image  1206  in the measurement image  1205  therein is obtained (S 1254 ). 
         [0058]      FIG. 12  illustrates the extracted patterns by binary images, but accuracy in the image positional alignment performed in step S 1252  can be improved in some cases by using a different image obtained by performing filtering processing on an original image of only regions of the hole pattern images  1204  and  1206  in the upper layer pattern images  1211  corresponding to the bole patterns  803  and  1003  and regions of the contact plug pattern images  1208  and  1210  in the lower layer pattern image  1212  corresponding to the contact plugs  804  and  1004  or an original image of only corresponding regions of the upper layer pattern images  1211  and the lower layer pattern images  1212 . 
         [0059]    For the lower layer pattern images  1212 , similarly to a case of the upper layer pattern images  1211 , image positional alignment between the reference image  1207  and the measurement image  1209  is performed (S 1253 ), and an amount (ΔLx, ΔLy) of positional misalignment between the contact plug pattern image  1208  in the reference image  1207  of the lower layer pattern images  1212  and the contact plug pattern image  1210  in the measurement image  1209  thereof is obtained (S 1255 ). Finally, using results obtained in S 1254  and  1255 , an overlay amount (Δx, Δy) is calculated (S 1256 ). 
         [0060]    The above method makes it possible to relatively measure how much positional relationship between the two patterns of the upper layer and the lower layer in the measurement image is misaligned with respect to positional relationship between the two patterns of the upper layer and the lower layer in the reference image. 
         [0061]    The description in  FIG. 12  refers to a case where the image has one pattern, but the image may have a plurality of patterns. In a case where the image has a plurality of patterns, the overlay amount illustrated in  FIG. 12  can be calculated on an individual pattern basis, and then an average value of the calculated amounts may be obtained. 
         [0062]    Another possible method is a method of collectively processing a plurality of patterns in an image. Specifically, a reference image and a measurement image including a plurality of patterns in upper layer pattern images are compared to each other, and an upper layer pattern positional misalignment amount is obtained. A reference image and a measurement image including a plurality of patterns in lower layer pattern images are compared to each other, and a lower layer pattern positional misalignment amount is obtained. Based on the obtained upper layer pattern positional misalignment amount and the obtained lower layer pattern positional misalignment amount, an overlay amount can be calculated. 
         [0063]      FIG. 13  illustrates procedures of processing of displaying results of the overlay measurement of  FIG. 12 . Differentiation filter processing is performed on the reference image  1201  and the measurement image  1202  targeted for the measurement, both illustrated in  FIG. 12  (S 1351 ) to generate a differential reference image  1301  including an upper layer hole pattern  1302  (also a lower layer contact plug pattern image  1302 ) and a differential measurement image  1303  including an upper layer hole pattern  1304  and a lower layer contact plug pattern image  1305 . For example, a Sobel filter is used as a differential filter. 
         [0064]    Next, the differential reference image  1301  is colored in color  1  (S 1352 ), and the differential measurement image  1303  is colored in color  2  (S 1353 ). The color  1  and the color  2  are different from each other, and colors providing excellent color contrast effect are selected. More specifically, the color  1  and the color  2  are in complementary relationship, or only R is used for the color  1  and a mixture of G and B is used for the color  2  in an RGB color model, or a mixture of G and B is used for the color  1  and only R is used for the color  2 . In the coloring of the differential reference image  1301 , a value obtained by subjecting a gray level of the differential reference image  1301  to linear conversion or non-linear conversion is set as an intensity of the color  1 . The same applies to the coloring of the differential measurement image  1303 . 
         [0065]    Next, image positional correction of the colored differential reference image  1301  is performed by using the lower layer pattern positional misalignment amount (ΔLx, ΔLy) calculated in step S 1255  in the process flow of  FIG. 12  (S 1354 ), and position of the lower layer contact plug pattern image  1302  (also corresponds to the upper layer hole pattern image) of the differential reference image  1301  is aligned with position of the lower layer contact plug pattern image  1305  of the differential measurement image  1303 . 
         [0066]    Finally, an image obtained by coloring the differential reference image  1301  in S 1352  and subjecting the colored differential reference image  1301  to the positional correction in S 1354  is superposed on an image obtained by coloring the differential measurement image  1303  in S 1353  (S 1356 ) to obtain a display image  1310 . The display image  1310  is obtained by emphasizing a pattern edge by the differential filter processing (S 1351 ) and mutually superposing the reference image and the measurement image colored in the different colors in S 1352  and S 1353 . The display image  1310  is displayed on a screen of the display terminal  756 . 
         [0067]    Numeral  1311  in the display image  1310  represents an outline of the lower layer contact plug pattern image  1302  in the differential reference image  1301  (also an outline of the upper layer hole pattern image  1302  in the differential reference image  1301 ), numeral  1312  represents an outline of the lower layer contact plug pattern image  1305  in the differential measurement image  1303 , and numeral  1313  represents an outline of the upper layer hole pattern image  1304  in the differential measurement image  1303 . The display image  1310  makes it possible to confirm that the positional alignment between the lower layer contact plug pattern image  1302  in the differential reference image  1301  and the lower layer contact plug pattern image  1305  in the differential measurement image  1303  is properly performed. 
         [0068]    For better visibility on the display image  1310 , the outline  1311  of the lower layer contact plug pattern image  1302  in the differential reference image  1301  and the outline  1312  of the lower layer contact plug pattern image  1305  in the differential measurement image  1303  are drawn with slight misalignment therebetween. On an actual image, the outlines  1311 ,  1312 , and  1313  in the display image  1310  are not thin lines as illustrated in the figure, but wide lines which are brightest at a pattern edge position and become darker with a distance therefrom. In accordance with the way of coloring described above, transmittance is ensured at a portion where any of the outlines  1311 ,  1312 , and  1313  are superposed on each other, thus making it easy to confirm a superposition state of the line patterns with a wide gray scale. 
         [0069]    In contrast, to confirm a result of the positional alignment between the upper layer hole pattern image  1302  in the differential reference image  1301  and the upper layer hole pattern image  1304  in the differential measurement image  1303 , by using the upper layer pattern positional misalignment amount (ΔUx, ΔUy) calculated in S 1254 , the positional alignment is performed on the differential reference image  1301  colored in S 1352  (S 1355 ), and then after position of the upper layer hole pattern image  1302  in the differential reference image  1301  is aligned with position of the upper layer hole pattern image  1304  in the differential measurement image  1303 , superposition on the differential measurement image  1303  colored in S 1353  is performed (S 1357 ) to obtain a display image  1315 . The display image  1315  is displayed on the screen of the display terminal  756 . 
         [0070]    The display image  1315  is an image obtained by emphasizing a pattern edge through the differential filter processing (S 1351 ) and superposing the differential reference image  1301  and the differential measurement image  1303  colored in the different colors in  1352  or  1353 . 
         [0071]    Numeral  1314  in the display image  1315  represents an outline of the upper layer hole pattern image  1302  in the differential reference image  1301  (also an outline of the lower layer contact plug pattern image), numeral  1312  represents an outline of the lower layer contact plug pattern image  1305  in the differential measurement image  1303 , and numeral  1313  represents an outline of the upper layer hole pattern image  1304  in the differential measurement image  1303 . The display image  1315  makes it possible to confirm that the positional alignment between the upper layer hole pattern image  1302  in the differential reference image  1301  and the upper layer hole pattern image  1304  in the differential measurement image  1303  is performed properly. 
         [0072]    For better visibility on the display image  1315 , the outline  1314  of the upper layer hole pattern image  1302  in the differential reference image  1301  and the outline  1313  of the upper layer hole pattern  1304  in the differential measured image  1303  are drawn with slight misalignment therebetween. 
         [0073]    Displaying the display image  1310  and the display image  1315  on the screen of the display terminal  756  for the confirmation makes it possible to recognize whether or not the positional alignment has been executed properly. If the positional alignment has been executed properly, it can be said that the upper layer pattern positional misalignment amount (ΔUx, ΔUy) and the lower layer pattern positional misalignment amount (ΔLx, ΔLy) are calculated properly and the overlay value calculated in S 1256  of  FIG. 12  is reliable. On the contrary, if there is any abnormal misalignment between the outlines (pattern edges)  1311 ,  1312 , and  1313  or between the outlines  1312 ,  1313 , and  1314  in the display image  1310  and the display image  1315 , the image positional alignment performed in  1252  and S 1253  of  FIG. 12  results in failure, and the overlay amount calculated in S 1256  includes a mistake. 
         [0074]    In the flow of the processing described in  FIG. 13 , the image positional correction is performed in S 1354  and  1355  after the coloring performed in S 1352 , but order of the aforementioned operations may be reversed so that the coloring may be performed in S 1352  after the image positional correction performed in S 1354  and  1355 . 
         [0075]    The flow of the processing described in  FIG. 13  has been described, referring to procedures of performing the positional correction of the differential reference image  1301  and superposing the resulting differential reference image  1301  on the differential measurement image  1303  to obtain the display images  1310  and  1315 , but the differential measurement image  1303  may be subjected to positional correction and superposed on the differential reference image  1301  to obtain display images. 
         [0076]      FIG. 14  illustrates a modified example of the procedures of the processing of displaying the results of the overlay measurement illustrated in  FIG. 13 . As a result of subjecting a reference image  1201  and a measurement image  1202  to differential filter processing (S 1403 ) as described in S 1351  of  FIG. 13 , as schematically illustrated on images  1411  and  1413 , in addition to patterns  1412  and  1414  targeted for measurement, a portion  1415  with a high differential value may appear. Creating a display image including such a portion deteriorates visibility, so that on the reference image  1201 , an image  1421  obtained by extracting an upper layer pattern of the reference image  1201  in a binary manner is subjected to inversion processing (S 1401 ) to thereby obtain an upper layer pattern mask image  1422 . A white portion  1423  of the upper layer pattern mask image  1422  is defined as 1 and a black portion  1424  thereof is defined as 0. The image  1411  can be subjected to mask processing (S 1403 ) by the upper layer pattern mask image  1422  to thereby erase the portion with the high differential value appearing in addition to the patterns targeted for the measurement and obtain a differential image  1425  corresponding to the image  1302  of  FIG. 13 . 
         [0077]    Similarly, on a measurement image  1202 , an image  1431  obtained by extracting an upper layer pattern of the measurement image  1202  in a binary manner is subjected to inversion processing (S 1402 ) to thereby obtain an upper layer pattern mask image  1432 . A white portion  1433  of the upper layer pattern mask image  1432  is defined as 1, and a black portion  1434  thereof is defined as 0. With the upper layer pattern mask image  1432 , an image  1413  obtained by subjecting the measurement image  1202  to differential filter processing in S 1351  can be subjected to mask processing (S 1404 ) to thereby obtain a differential image  1435  corresponding to the image  1303  of  FIG. 13  with the erased portion with the high differential value appearing in addition to the patterns targeted for the measurement. 
         [0078]    Processing thereafter permits subjecting the image  1425  to the processing in and after the coloring processing S 1352  illustrated in  FIG. 13  and subjecting the image  1435  to the processing in and after the coloring processing S 1353  to thereby obtain display images  1310  and  1315  having a 0 value in regions other than measurement regions of the reference image  1201  and the measurement image  1202 . In such a case, the regions other than the measurement regions are black, thus making it possible to clearly display the images, such as the hole patterns  803  and  1003  and the contact plugs  804  and  1004 , at portions targeted for the measurement. A value of a portion to be masked is not limited to 0 and it can be replaced with a different value and a different color. 
         [0079]      FIG. 15  illustrates a state in which the display images  1310  and  1315  described with reference to  FIG. 13  are superposed on each other. The image  1302  (an outline on the hole pattern image) in the differential reference image  1301  described in  FIG. 13  is used, in a state illustrated in  FIG. 15 , as the pattern  1314  for positional alignment with the upper layer pattern  1313  of the differential measurement image  1303  and as the pattern  1311  for positional alignment with the lower layer pattern  1312  of the differential measurement image  1303 . Thus, as a result of defining a representative position of the hole pattern image  1302  of the differential reference image  1301 , a change in a representative position  1502  (a central position of a pattern  1314  in the example illustrated in  FIG. 15 ) of the hole pattern of the differential reference image  1301  obtained upon the positional alignment with the upper layer pattern of the differential measurement image  1303  and a representative position  1501  (a central position of the pattern  1311  in the example illustrated in  FIG. 15 ) of the hole pattern of the differential reference image  1301  obtained upon the positional alignment with the lower layer pattern of the differential measurement image  1303  is directly provided as an overlay amount. Therefore, a vector display  1601  obtained by linking together the aforementioned points  1501  and  1502  as illustrated in  FIG. 16  can display an amount and a direction of the aforementioned misalignment. 
         [0080]    A possible way of defining the representative position of the hole pattern of the reference image includes, for example, pixel value weighing of the reference image subjected to differential filter processing and masking. The representative position may be a position which corresponds to a partial region of the image and which is fixed relatively to coordinates of a measurement window set for performing the overlay measurement processing. However, the representative position is not limited to the aforementioned position and may be defined as desired. 
         [0081]      FIG. 17  illustrates superposition of the measurement image  1202  itself, instead of the differential measurement image subjected to the differential filter processing and the coloring processing, on the display image  1310  of  FIG. 13 . This makes it possible to confirm with which position the pattern edge  1311  of the reference image  1201  has been aligned on the original image of the measurement image  1202  on which a lower layer pattern image  12021  and a lower layer image  12022  are displayed. In such a case, for the measurement image  1202  superposed in the image superposition step S 1356 , gray values of the original measurement image may be evenly assigned to R, G, and B values, and as intensity of saturation of a specific color, the assignment may be achieved by subjecting gray values of the reference image subjected to the differential filter processing to linear conversion or non-linear conversion. 
         [0082]      FIG. 18  illustrates superposition of the reference image  1201  itself on which a lower layer pattern image  12011  whose position is aligned with the pattern edge  1311 , instead of the differential reference image subjected to the differential filtering processing and the coloring processing, on the display image  1310  of  FIG. 13 . The same substitute display method is also applicable to the display image  1315  of  FIG. 13 . 
         [0083]    Switching of display elements makes it easier to confirm the results of the overlay measurement. The display elements here in  FIG. 13  refer to seven elements including: (1) the differential reference image obtained after the image positional correction processing (S 1354 ); (2) the differential reference image obtained after the image positional correction processing (S 1355 ); (3) the differential measurement image obtained after the coloring processing (S 1353 ); (4) the vector display  1601  of  FIG. 16 ; (5) the differential reference image subjected to the positional correction (S 1354 ) with the lower layer pattern positional misalignment amount (ΔLx, ΔLy); (6) the differential reference image subjected to the positional correction (S 1355 ) with the upper layer pattern positional misalignment amount (ΔUx, ΔUy); and (7) the measurement image. Sequentially changing at least any desired one of the display elements, a superposition thereof, or a combination of superposition thereof makes it easier to confirm the results of the overlay measurement. For example, constantly displaying (3) the differential measurement image obtained after the coloring processing (S 1353 ) and performing confirmation while switching between display and non-display of (1) the reference image obtained after the image positional correction processing (S 1354 ) makes it possible to easily evaluate whether or not position of the lower layer pattern  902  of the reference image is properly aligned with position of the lower layer pattern  1102  of the measurement image. 
         [0084]    The method of performing the image positional correction on the reference image based on the lower layer pattern positional misalignment amount or the upper layer pattern positional misalignment amount to perform superposed display of the images has been described above, but it is obvious that positional correction can be performed on the measurement image based on the lower layer pattern positional misalignment amount or the upper layer pattern positional misalignment amount to perform superposed display of the images. In such a case, it should be noted that the lower layer pattern positional misalignment amount or the upper layer pattern positional misalignment amount is reversely directed between a case where the reference image is subjected to the image positional correction and a case where the measurement image is subjected to the image positional correction. 
         [0085]      FIG. 19  illustrates a flow of processing performed for carrying out, with the overlay measurement device  1000  illustrated in  FIG. 7 , the method of displaying the results of the overlay measurement described with reference to  FIG. 12  to  FIG. 18 . 
         [0086]    First, a pattern serving as a reference is imaged with the SEM to acquire a reference image, and the obtained image is then stored as the reference image  1201  into a storage region of the image processing section  752  (S 1901 ). Next, the semiconductor wafer  707  having a circuit pattern targeted for overlay measurement is loaded onto the scanning electron microscope device  700  of the overlay measurement device  1000  illustrated in  FIG. 7 , and is then placed on the stage  706  (S 1902 ). After the loading of the semiconductor wafer  707 , the stage  706  is controlled through the stage controller  755  by the overall control section  753  of the processing and control section  750 , and the stage  706  is moved in a manner such that a section of the measurement patterns on the semiconductor wafer  707  without any misalignment falls in an observation visual field of the irradiation optical system  710  of the scanning electron microscope device  700  (S 1903 ). 
         [0087]    Next, under control of the deflection coils  704  by the electron optical system control section  754 , a region (region targeted for the measurement) including the pattern targeted for the overlay measurement and formed on the semiconductor wafer  707  is scanned by the electron beam  701 . A signal obtained by detecting, with the detector  708 , the secondary electron generated from the region targeted for the measurement is converted in a digital signal by the A/D converter  709 , is inputted as a photographed image into the image processing section  710 , and is stored into a memory (not illustrated) of the image processing section  710  (S 1904 ). 
         [0088]    Subsequently, the reference image  1201  and the measurement image  1202  are read from the memory of the image processing section  710 , and the overlay (Δx, Δy) illustrated in  1212  is calculated at a calculation section (not illustrated) of the image processing section  710  in accordance with the processing flow illustrated in  FIG. 12  (S 1905 ). After ending of the calculation processing, the upper pattern positional misalignment amount (ΔUx, ΔUy) illustrated in S 1254  of  FIG. 12 , the lower layer pattern positional misalignment amount (ΔLx, ΔLy) illustrated in S 1255  and the overlay amount (Δx, Δy) illustrated in S 1256  are stored into the memory of the image processing section  710 . 
         [0089]    For display of the results, the reference image  1201 , the measurement image  1202 , the upper layer pattern positional misalignment amount  1210 , and the lower layer pattern positional misalignment amount  1211  are read, the processing flow illustrated in  FIG. 13  is executed at the calculation section of the image processing section  710 , and the display images  1310  and  1315  are calculated and stored into the memory of the image processing section  710  (S 1906 ). The calculated display images  1310  and  1315  are outputted from the image processing section  710  to the overall control section  713 , which displays the display image  1310  or the display image  1315  at the display terminal  714  (S 1907 ). Steps S 1903  to S 1907  are executed for all measurement points. 
         [0090]    To partially display only the measurement region at the display terminal  714 , upon execution of the processing illustrated in  FIG. 12 , the upper layer pattern reference image  1204  and the upper layer pattern measurement image  1205  are further stored into the memory of the image processing section  710 . In the display of the results, the reference image  1201 , the measurement image  1202 , the upper layer pattern reference image  1204 , the upper layer measurement image  1205 , the upper layer pattern positional misalignment amount  1210 , and the lower layer pattern positional misalignment amount  1211  are read. At the calculation section of the image processing section  710 , the processing flow illustrated in  FIG. 13  and  FIG. 14  is executed using the read information, and the display image  1310  or the display image  1315  is calculated and stored into the memory of the image processing section  710 . The calculated display image  1310  or  1315  is outputted from the image processing section  710  to the overall control section  713 , which displays the display image  1310  or the display image  1315  at the display terminal  714 . 
         [0091]    For the vector display illustrated in  FIG. 16 , the representative position of the hole pattern of the reference image is calculated at the calculation section of the image processing section  710  described above, and is stored into the memory of the image processing section  710  in correspondence with the reference image. Upon the overlay measurement, the aforementioned overlay amount Δx, Δy and the representative position of the reference image which are stored in the memory of the image processing section  710  are read by the overall control section  713 , a vector display of the overlay amount focused on the representative position is generated at a calculation section (not illustrated) of the overall control section  713 , and is stored into a memory (not illustrated) of the overall control section  713  and displayed at the display terminal  714 . 
         [0092]    In the embodiment described above, the reference image is previously recorded, but may be acquired each time from the semiconductor wafer targeted for the measurement. 
         [0093]      FIG. 20  illustrates a detailed example of the display provided at the display terminal  714 . An ID  20001  refers to an identification number of a reference image  2003 , a measurement image  2004 , or a resulting measurement image  2005 , and corresponds to an overlay measurement section on the wafer. Numeral  2002  represents a numerical value of overlay measurement results.  FIG. 20  also displays the reference image  2003  and the measurement image  2004  together with the resulting measurement image  2005 , but may not display either or both of the aforementioned images when not necessary. Contents of the display of the resulting measurement image  2005  is switched by measurement results display switching  2006 . The measurement results display switching  2006  includes switches respectively located aside of “Image comparison results”, “Vector display”, “Reference image”, “Measurement image”, “Reference image mask”, and “Measurement image mask”, with black circles each indicating that the corresponding item has been selected and white circles each indicating that the corresponding item has not been selected. Switching between Selected and Not selected is performed through, for example, clicking performed on the screen. Hereinafter, each of the items displayed in the measurement results display switching  2006  will be described. 
         [0094]    In the item “Image comparison results”, in a case where the upper layer is selected, based on the positional misalignment amount of the upper layer pattern  1210 , the reference image is subjected to positional correction and superposed on the measurement image to be displayed as the resulting measurement image  2005 , and in a case where the lower layer is selected, based on the positional misalignment amount of the lower layer pattern  1211 , the reference image is subjected to positional correction and superposed on the measurement image to be displayed as the resulting measurement image  2005 , and either or both of the upper layer and the lower layer can be selected. The display image  1315  is a display example when only the upper layer is selected, and the display image  1310  is a display example when only the lower layer is selected. 
         [0095]    The item “Vector display” is an item for selecting whether or not to superpose the vector display illustrated in  FIG. 16  on the resulting measurement image  2005 . 
         [0096]    The item “Reference image” is an item for selecting whether the reference image to be superposed on the resulting measurement image  2005  is provided as an original image or a colored edge filter image. In a case where the both are not selected, the reference image is not superposed on the resulting measurement image  2005 . 
         [0097]    The item “Measurement image” is an item for selecting whether the measurement image to be superposed on the resulting measurement image  2005  is provided as an original image or a colored edge filter image. In a case where the both are not selected, the measurement image is not superposed on the resulting measurement image  2005 . 
         [0098]    The item “Reference image mask” is an item for selecting whether or not to mask, by the upper layer pattern reference image  1204 , the reference image to be superposed on the resulting measurement image  2005 . 
         [0099]    The item “Measurement image mask” is an item for selecting whether or not to mask, by the upper layer pattern measurement image  1204 , the measurement image to be superposed on the resulting measurement image  2005 . 
         [0100]    In accordance with input of the measurement results display switching described above, the overall control section  713  reads the information stored in the memory of the image processing section  710  or the memory of the overall control section  713 , updates display contents of the resulting measurement image  2005 , and outputs the updated contents to the display terminal  714 . 
         [0101]    As described above, with the present Example, in the overlay measurement using the SEM image and using the product circuit of the semiconductor device through comparison between the reference image and the measurement image, when the reference image or the measurement image is displayed in a superposed manner with misalignment in accordance with the obtained overlay amount, it is possible to improve visibility of the superposed display. 
       REFERENCE SIGNS LIST 
       [0000]    
       
         
           
               700  . . . Scanning electron microscope device, 
               701  . . . Electron beam, 
               702  . . . Electron gun, 
               703  . . . Capacitor lens, 
               704  . . . Deflection coil, 
               705  . . . Objective lens, 
               706  . . . Stage, 
               707  . . . Wafer, 
               708  . . . Detector, 
               750  . . . Processing and control section, 
               751  . . . A/D converter, 
               752  . . . Image processing section, 
               753  . . . Overall control section, 
               754  . . . Electron optical system control section, 
               755  . . . Stage controller, 
               756  . . . Display terminal, 
               1000  . . . Overlay measurement device.