Abstract:
Disclosed herein is a scanning electron microscope capable of determining quality of a semiconductor pattern, deformation of contact-holes, and inclination of the sidewall of the contact-holes, respectively. To that end, the scanning electron microscope includes image pickup means for picking up images of a circuit pattern formed on a semiconductor wafer on a preset condition, calculation means for comparing each of the images picked up by the image pickup means with a prestored reference image to thereby calculate a feature of the images picked up, and a computer for executing evaluation on quality of the circuit pattern on the basis of the feature calculated by the calculation means, and calculation of the feature is executed independently with reference to a secondary electron image, and each of back-scattering electron images.

Description:
FIELD OF THE INVENTION 
     The invention relates to a scanning electron microscope provided with means for automatically picking up images of a circuit pattern formed on a semiconductor wafer on a preset condition, and digitizing quality of a semiconductor pattern by use of the images as picked up, and a reference image to thereby calculate the same. 
     BACKGROUND OF THE INVENTION 
     It is necessary to inspect a pattern on a semiconductor wafer, formed in a semiconductor manufacturing process, to find out whether or not the pattern is formed as designed. Accordingly, there has since been developed a critical dimension SEM that is a scanning electron microscope with addition of a function for measuring dimensions of a pattern, thereby calculating features such as a pattern width, respective diameters of contact holes, roughness, and so forth, on the basis of image signals (refer to, for example, JP-A-2004-247394, and US2004/0156223A). Further, there have been practiced not only observation of an image, by use of the scanning electron microscope, but also automatic pickup, and classification of the image having various defects such as a pattern open, a pattern bridge, scratch, particle, and so forth, by use of a review SEM with addition of a function for determining the features of the image with the use of a computer. Still further, there has also been practiced fixed-point observation of critical parts of a semiconductor pattern, by use of the review SEM, to thereby display an image as picked up, and execute quantification of the quality of the pattern by calculating the features of the image. Furthermore, as progress is made in the miniaturization of the pattern, it has since been required to speedily, accurately, and simply calculate a quantitative value of the quality, including a three-dimensional shape such as the sidewall of the pattern, and the sidewall of each of the contact holes. 
     However, with a conventional technology, since the features of an image are calculated by use of only an image formed by secondary electrons ejected from a direction vertical to a wafer, it has been difficult to turn three-dimensional information such as the sidewall of the pattern, and the sidewall of each of the contact holes into an image, and even if imaging thereof is possible, it has been impossible to find the quantitative value of the quality. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to provide a scanning electron microscope capable of determining quality of a semiconductor pattern, deformation of contact-holes, and inclination of the sidewall of the contact-holes, respectively. 
     To that end, the invention provides in its one aspect a scanning electron microscope including image pickup means for picking up images of a circuit pattern formed on a semiconductor wafer on a preset condition, calculation means for comparing each of the images picked up by the image pickup means with a prestored reference image to thereby calculate a feature of the images picked up, and a computer for executing evaluation on quality of the circuit pattern on the basis of the feature calculated by the calculation means, wherein calculation of the feature is executed independently with reference to a secondary electron image, and each of back-scattering electron images. 
     Thus, with the invention, it is possible to obtain the scanning electron microscope capable of accurately and easily determining quality of the semiconductor pattern, deformation of contact-holes, and inclination of the sidewall of the contact-holes, respectively. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a vertical sectional view broadly showing a configuration of a scanning electron microscope; 
         FIG. 2  is a flowchart showing a basic flow of operational steps according to the invention; 
         FIG. 3  is a histogram of an image, showing a relationship of pixel value vs. pixel numbers; 
         FIGS. 4(   a ) and  4 ( b ) each are a screen view showing an example of an image displayed on a screen of the scanning electron microscope; and 
         FIG. 5  is a screen view showing an example of an image of a pattern, displayed by the scanning electron microscope. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of a scanning electron microscope according to the invention is described hereinafter with reference to the accompanying drawings.  FIG. 1  is a vertical sectional view broadly showing a configuration of the scanning electron microscope. The scanning electron microscope  100  includes an enclosure  101  containing a table  103  for transferring a wafer  102 . An electron beam  104  irradiated from an electron beam source  118  is two-dimensionally scanned over the wafer  102  by a deflector  105 . The focusing of the electron beam  104  is adjusted by an electron lens  106 . 
     By scanning over the wafer, with the electron beam, secondary electrons  107 , and back-scattering electrons  108  are ejected from the wafer. Those secondary electrons  107 , and back-scattering electrons  108 , as ejected, are detected by detectors  109 ,  110 , respectively, to be converted into electrical signals, and analog signals are further converted into digital signals, respectively, by A/D converters  111 , thereby enabling a two-dimensional gray scale image to be obtained. The image as obtained is delivered as image signals to a processor  112  where processing, such as calculation of features of the image, is executed. Results of the processing are shown on a display  116  of a computer  114 , and are transmitted as necessary to a memory such as a hard disk  113 , or to a computer  117  of another system via a network. Overall control is executed by the computer  114 , and control of operations of a system is executed by a computer  115  for control. An image formed on the basis of the secondary electrons  107  is referred to as an SE image, and an image formed on the basis of the back-scattering electrons  108  is referred to as a BSE image. 
       FIG. 2  is a flowchart showing a basic flow of operational steps according to the invention. The operational steps are pre-stored in the hard disk  113 , and are executed by the computer  114 . First, there are set conditions on which an image of a circuit pattern formed on a semiconductor wafer is automatically picked up (step  201 ). The conditions set at this point in time include an addressing image for transferring a field of view to a pattern image to be picked up, an acceleration voltage and a probe current of the electron microscope, size of the field of view for a picked-up image, the number of summed up sheets of images, an auto-focusing condition, and so forth. Further, if the number of images to be picked up is plural, chip numbers and chip positions, for identifying respective locations of the images are designated. Furthermore, a reference image is designated in order to calculate features of images. 
     Next, images at a desired location are picked up on the conditions described above (step  202 ). Picked up images are two sheets of the BSE images, acquired from two units of the detectors  110  opposing each other, respectively, besides the SE image. The SE image is normally an image acquired when the wafer is observed from the direction vertical thereto, and each of the BSE images is a shaded image that is acquired when the wafer is irradiated with light from a direction slanting thereto, and is observed from the direction vertical thereto. 
     Subsequently, by making use of three sheets of the images picked up as above, the features of the pattern are calculated. For calculation of the features, use is made of a normalized correlation value R against the reference image. 
                   R   =       ∑       (     f   ij     )     ×     (     g   ij     )             ∑         (     f   ij     )     2     ×       (     g   ij     )     2                     Expression   ⁢           ⁢     (   1   )                 
where f ij : reference image, g ij : picked-up image Herein, since the three sheets of the images are used, three pieces of the normalized correlation values can be obtained. Those normalized correlation values are designated R SE , R L , and R R , respectively. Those normalized correlation values R each represents similarity between the reference image f, and the picked-up image g, serving as an index for quantification of the quality of the pattern. Because only the SE image has been in use for expression of the similarity in the past, only two-dimensional information has been directly obtainable, so that it has been impossible to quantify the quality including the three-dimensional shape such as the sidewall of the pattern.
 
     With the invention, since the shape of the sidewall of the pattern can be portrayed as information on the shaded image with the use of the BSE images, the three-dimensional shape of the pattern can be quantified according to the feature referred to as the similarity by calculating the normalized correlation values of R L , and R R  of the BSE images, respectively, thereby enabling a three-dimensional quality to be evaluated. 
     Finally, the feature R calculated above, picked-up images, information with the picked-up images attached thereto, and so forth are shown in the display  116  (step  203 ). Further, those results are transmitted as necessary to the memory such as the hard disk  113 , and to the computer  117  of another system via the network. Further, there can be the case where use is made of a composite image made up of the SE image and the BSE images depending on the features as calculated.
 
 f   R   ′=αf   SE   +βf   R  
 
 f   L   ′=αf   SE   +βf   L    Expression (2)
 
where α, β each are a constant
 
     If, for example, the expression (2) holds, the feature can be calculated on the basis of an image acquired by adding information on a shaded image from the left side or the right side to information on the shape as observed from the vertical direction, thereby enabling quality including the three-dimensional shape of the pattern to be quantified. 
     Further, the preceding expression may be as follows:
 
 f   R   ′=αf   R   +βf   L  
 
 f   L   ′=αf   L   +βf   R    Expression (3)
 
where α, β each are a constant
 
     According to the expression described as above, it is possible to calculate the feature on the basis of an image with emphasis on either the left-side shaded image, or the right-side shaded image, so that quality with more emphasis on the three-dimensional information concerning the sidewall of the pattern can be quantified. 
     Further, image synthesis is also possible as follows:
 
 f′=αf   SE   +βf   R   +γf   L    Expression (4)
 
where α, β, and γ each are a constant
 
     According to the expression described above, it follows that one sheet of an image includes information on both the left-side and the right-side shaded images, so that it is possible to quantify the quality of the pattern, according to a three-dimensional shape thereof. 
     Only the highlighted part, and shadow part are extracted through threshold-value processing from the image acquired from the expression (3), and subsequently, the feature can also be calculated by use of those extracted images. In order to do so, against the image f R′ =αf R +βf L  as acquired from the expression (3), there is first decided a threshold value at which only the highlight part or shadow part can be extracted. 
       FIG. 3  is a histogram of an image, showing a relationship of pixel value vs. pixel numbers. Normally, the histogram  300  of the image, as shown in  FIG. 3 , is worked out, thereby deciding threshold values  303 , or  304 , respectively, at which either a highlight part  301  larger in pixel value or a shadow part  302  smaller in pixel value can be separated. If the pixel numbers have three peaks, as in the case of an example shown in  FIG. 3 , it will suffice to provide the threshold values at two valleys, respectively. Because the threshold values as decided herein are normally applicable to other images as well, if the threshold values are preset as image pickup conditions, it becomes unnecessary for an operator to set the threshold values on an image-by-image basis. If the threshold values are dependent on a wafer and the image pickup conditions, the threshold values are kept registered in recipes corresponding thereto, respectively. If a plurality of the threshold values are kept registered, it is possible to make selection therefrom as appropriate. 
       FIGS. 4(   a ) and  4 ( b ) each are a screen view showing an example of an image displayed on a screen of the scanning electron microscope, in which  FIG. 4(   a ) displays a highlight part  401  separated at the threshold value  303  shown in  FIG. 3 , and  FIG. 4(   b ) displays a shadow part  402  separated at the threshold value  304  shown in  FIG. 3 . Thus, if the histogram shown in  FIG. 3  is separated into parts at the respective threshold values, the highlight part  401  and the shadow part  402  of the image, in as-separated state, will be displayed, thereby enabling the quality of a three-dimensional image to be quantified with ease. 
     As an image for use in calculation of the feature, use can be made of only a designated range of a picked-up image instead of using the picked-up image in whole. 
       FIG. 5  is a screen view showing an example of an image of a pattern, displayed by the scanning electron microscope, in which a line-space pattern is displayed within an image  500 . When the quality of the pattern is evaluated with attention being focused on a defective spot  501  of the pattern, it is unnecessary to evaluate quality of a region surrounding the defective spot  501 . Accordingly, a program is preset in the hard disk  113  such that the quality of the interior of only a region  502  for evaluating quality, designated on the screen, is quantified by the computer  114 . 
     The region  502  may be manually set on the image-by-image basis by the operator, however, when the same defect as is found in the defective spot  501  is to be detected from a plurality of images with attention being focused on occurrence of the defective spot  501 , an image of the region  502  is kept registered as a template in the hard disk  113 , and use of the template as a feature calculation area at the time of setting the image pickup conditions is designated, causing the computer  114  to automatically detect the area out of a picked-up image by template matching processing to thereby calculate the feature. For example, if the sidewall of the pattern, obtained from an image acquired by tilting an electron beam, is designated as the region  502 , this will eliminate the need for designating the sidewall every time an image is acquired, thereby enabling evaluation on the quality of the sidewall to be implemented with ease. 
     As described in the foregoing, with the use of the image formed on the basis of the back-scattering electrons obtained from the oppositely disposed detectors, in addition to the image formed by the secondary electrons from the direction vertical to the wafer, information on the sidewall of a pattern can be visualized as a shaded image; and the feature is calculated as a quantitative value from the information on the sidewall, thereby enabling the three-dimensional quality of a semiconductor pattern to be quantified. 
     
       FIG. 2 
         
           201 : setting of image pickup conditions 
           202 : image pickup (repeated for a set number of times) 
           203 : results shown on a display 
                   : pixel numbers 
                   : pixel value