Abstract:
A method for measuring overlay includes receiving a first image of a first overlay mark captured using light having a first wavelength. The method includes receiving a second image of a second overlay mark captured using light having a second wavelength different from the first wavelength. The method includes measuring a displacement between a central portion of the first image and a central portion of the second image, wherein the first and second overlay marks are disposed on different levels.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This U.S. nonprovisional patent application is a continuation of U.S. patent application Ser. No. 14/182,697, filed on Feb. 18, 2014 which claims priority under 35 U.S.C. §119 to Korean Patent Application No. 10-2013-0025091 filed on Mar. 8, 2013, the disclosures of which are incorporated by reference herein in their entireties. 
     
    
     TECHNICAL FIELD 
       [0002]    The present inventive concept relates to a method for measuring overlays and, more particularly, to a method for measuring overlay errors using image based overlay measurement techniques. 
       DISCUSSION OF RELATED ART 
       [0003]    In semiconductor manufacturing processes, there is a need for measuring and controlling specific wafer parameters. Overlay error is one of the wafer parameters. Overlay error can be referred to as a relative displacement between structures formed on different layers in the wafer. The larger the overlay error is between structures, the greater the misalignment is between the structures. The yield and performance of semiconductor devices can be decreased due to overlay error. 
       SUMMARY 
       [0004]    Exemplary embodiments of the present inventive concept provide a method for measuring overlay errors using different wavelengths. 
         [0005]    Exemplary embodiments of the present inventive concept provide a method for measuring overlay errors in which an image corresponding to overlay marks formed on a lower layer is acquired using light having a longer wavelength and an image corresponding to another overlay mark formed on an upper layer is acquired using light having a shorter wavelength. 
         [0006]    According to an exemplary embodiment of the present inventive concept, a method for measuring overlay includes receiving a first image of a first overlay mark captured using light having a first wavelength and a second image of a second overlay mark captured using light having a second wavelength different from the first wavelength. The method for measuring overlay includes measuring a displacement between a central portion of the first image and a central portion of the second image, wherein the first overlay mark and the second overly mark are disposed on different levels. 
         [0007]    According to an exemplary embodiment of the present inventive concept, the first overlay mark may be provided on a lower level than the second overlay mark, and the first wavelength may be longer or shorter than the second wavelength. 
         [0008]    According to an exemplary embodiment of the present inventive concept, at least one of the first and second wavelengths may be included in a range of visible light, a range above the range of visible light, or a range below the range of visible light. 
         [0009]    According to an exemplary embodiment of the present inventive concept, the first overlay mark may be disposed on a first layer of a wafer, and the second overlay mark may be disposed on a second layer over the first layer of the wafer. 
         [0010]    According to an exemplary embodiment of the present inventive concept, the first and second overlay marks may be provided on a scribe lane of the wafer. 
         [0011]    According to an exemplary embodiment of the present inventive concept, a method for measuring overlay may include receiving a first image corresponding to a first overlay mark captured using light having a first wavelength, the first overlay mark disposed on a first layer of a wafer. The method for measuring overlay may include receiving a second image corresponding to a second overlay mark captured using light having a second wavelength different from the first wavelength, the second overlay mark disposed on a second layer over the wafer. The method for measuring overlay may include receiving a combined image in which the first and second images are overlapped. The method for measuring overlay may include calculating a displacement between a central portion of the first image and a central portion of the second image in the combined image to measure an overlay between the first overlay mark and the second overlay mark. 
         [0012]    According to an exemplary embodiment of the present inventive concept, the first wavelength may be longer or shorter than the second wavelength. 
         [0013]    According to an exemplary embodiment of the present inventive concept, the method may further include receiving a third image corresponding to a third overlay mark captured using light having a third wavelength. The third overlay mark may be provided on a third layer over the second layer. The third wavelength may be different from the first and second wavelengths. 
         [0014]    According to an exemplary embodiment of the present inventive concept, the first wavelength may be longer or shorter than the second wavelength, and the second wavelength may be longer or shorter than the third wavelength. 
         [0015]    According to an exemplary embodiment of the present inventive concept, the first overlay mark may be disposed on the first layer that corresponds to a scribe lane of the wafer. The second overlay mark may be disposed on the second layer that corresponds to the scribe lane of the wafer. 
         [0016]    According to an exemplary embodiment of the present inventive concept, one of the first and second overlay marks may be horizontally spaced apart from the other and may be not vertically overlapped with the other. 
         [0017]    According to an exemplary embodiment of the present inventive concept, the second layer may be directly on the first layer, or an additional layer may be further disposed between the first and second layers. 
         [0018]    According to an exemplary embodiment of the present inventive concept, the first overlay mark may include a plurality of first parallel lines that are spaced apart. The second overlay mark may include a plurality of second parallel lines that are spaced apart. 
         [0019]    According to an exemplary embodiment of the present inventive concept, receiving the first image may include selecting the first parallel lines and obtaining an image of the first parallel lines using the light having the first wavelength. 
         [0020]    According to an exemplary embodiment of the present inventive concept, capturing the second image may include selecting the second parallel lines and obtaining an image of the second parallel lines using the light having the second wavelength. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    The above and other features of the inventive concept will become more apparent by describing in detail exemplary embodiments thereof, with reference to the accompanying drawings in which: 
           [0022]      FIG. 1  is a top plan view of an overlay mark according to an exemplary embodiment of the present inventive concept; 
           [0023]      FIG. 2  is a flow diagram illustrating a method for measuring overlay according to an exemplary embodiment of the present inventive concept; 
           [0024]      FIGS. 3A to 3F  are top plan views illustrating a method for measuring overlay according to an exemplary embodiment of the present inventive concept; 
           [0025]      FIG. 3D  is a top plan view illustrating a portion of  FIG. 3C ; 
           [0026]      FIG. 3F  is a top plan view illustrating a portion of  FIG. 3E ; 
           [0027]      FIG. 4  is a diagram of an overlay measurement system according to an exemplary embodiment of the present inventive concept; 
           [0028]      FIG. 5  is a top plan view of an overlay mark according to an exemplary embodiment of the present inventive concept; and 
           [0029]      FIG. 6  is a top plan view of an overlay mark according to an exemplary embodiment of the present inventive concept. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0030]    Exemplary embodiments of the present inventive concept will now be described more fully hereinafter with reference to the accompanying drawings, in which some exemplary embodiments of the present inventive concept are shown. However, the present inventive concept should not be construed as limited to the exemplary embodiments set forth herein and may be embodied in different forms. 
         [0031]    In the drawings, the thicknesses of layers and regions may be exaggerated for clarity. Like reference numerals in the drawings and specification may denote like elements. 
         [0032]      FIG. 1  is a top plan view of an overlay mark according to an exemplary embodiment of the present inventive concept. 
         [0033]    Referring to  FIG. 1 , an overlay mark  100  may be formed on a scribe lane of a wafer to be used for determining an overlay between two or more layers stacked on the wafer or between two or more separate patterns on a single layer of the wafer. For ease of discussion, the overlay mark  100  may be mainly used to determine an overlay between two or more layers stacked on a wafer. It should be noted, however, that this is not a limitation and that the overlay mark  100  may also be used to determine an overlay between two or more separate patterns on a single layer of a wafer. 
         [0034]    The overlay mark  100  may be arranged within an optical perimeter  105  set by a field of view that defines an area available for capturing an image by a metrology tool used to measure overlay. The overlay mark  100  may comprise a plurality of zones configured to determine overlay errors in X and Y directions between two layers on the wafer. For example, the overlay mark  100  may comprise first zones  111 ,  112 ,  113  and  114  that may be provided on a first layer of the wafer and second zones  121 ,  122 ,  123  and  124  that may be provided on a second layer over the first layer of the wafer. The first zones  111  to  114  may be horizontally spaced apart from the second zones  121  to  124  and do not vertically overlap the second zones  121  to  124 . The second zones  121  to  124  may be positioned closer to a center (represented by a cross) of the optical perimeter  105  than the first zones  111  to  114 . 
         [0035]    The first zones  111  to  114  may include a first pattern  111 , a second pattern  112 , a third pattern  113 , and a fourth pattern  114  which are rotationally symmetric, for example +90°, 180°, 270°, 360° around the center of the optical perimeter  105 . Each of the first to fourth patterns  111  to  114  may include a plurality of first overlay marks  110 . The first overlay marks  110  may be a plurality of parallel lines arranged periodically on the first layer of the wafer. The first overlay marks  110  included in the first and third patterns  111  and  113  may be provided to measure an overlay in the Y direction while the first overlay marks  110  included in the second and fourth patterns  112  and  114  may be provided to measure an overlay in the X direction. 
         [0036]    Similarly, the second zones  121  to  124  may include a first pattern  121 , a second pattern  122 , a third pattern  123 , and a fourth pattern  124  which are rotationally symmetric, for example ±90°, 180°, 270°, 360° around the center of the optical perimeter  105 . Each of the first to fourth patterns  121  to  124  may include a plurality of second overlay marks  120 . The second overlay marks  120  may be a plurality of parallel lines arranged periodically on the second layer of the wafer. In exemplary embodiments of the present inventive concept, the second layer may be disposed directly on the first layer. An additional layer (e.g., additional layer  32  of  FIG. 4 ) may be further disposed between the first and second layers. The second overlay marks  120  included in the first and third patterns  121  and  123  may be provided to measure an overlay in the Y direction while the second overlay marks  120  included in the second and fourth patterns  122  and  124  may be provided to measure an overlay in the X direction. 
         [0037]      FIG. 2  is a flow diagram illustrating a method for measuring overlay according to an exemplary embodiment of the present inventive concept. 
         [0038]    Referring to  FIG. 2 , a method for measuring an overlay between the first and second layers of the wafer may begin at a first step S 110  where a first image (e.g.,  110   p  of  FIG. 3A ) corresponding to the first zones  111  to  114  is captured using light having a first wavelength. After obtaining the first image  110   p,  the process may proceed to a second step S 120  where a second image (e.g.,  120   p  of  FIG. 3B ) corresponding to the second zones  121  to  124  is captured using light having a second wavelength. After obtaining the second image  120   p,  the process may proceed to a third step S 130  where a combined image (e.g.,  130   p  of  FIG. 3C ) of the first and second images  110   p  and  120   p  is acquired. After obtaining the combined image  130   p,  the process may proceed to a fourth step S 140  where an overlay error is measured by calculating a displacement or offset between a center (e.g.,  110   c  of  FIG. 3A ) of the first image  110   p  and a center (e.g.,  120   c  of  FIG. 3B ) of the second image  120   p.  When an overlay error is present, a step S 150  may be performed to correct the overlay error. After the overlay correction, the first to fourth steps S 110  to S 140  may be selectively repeated. The overlay measurement may be further described in detail below. 
         [0039]      FIGS. 3A to 3F  are top plan views illustrating a method for measuring overlay according to an exemplary embodiment of the present inventive concept.  FIG. 3D  is a top plan view illustrating a portion of  FIG. 3C .  FIG. 3F  is a top plan view illustrating a portion of  FIG. 3E .  FIG. 4  is a diagram of an overlay measurement system according to an exemplary embodiment of the present inventive concept. 
         [0040]    Referring to  FIGS. 1 and 3A , a first image  110   p  may be acquired by selectively capturing the first zones  111  to  114  each having the first overlay marks  110  from the overlay mark  100 . The first image  110   p  may include a first pattern image  111   p  corresponding to the first pattern  111 , a second pattern image  112   p  corresponding to the second pattern  112 , a third pattern image  113   p  corresponding to the third pattern  113 , and a fourth pattern image  114   p  corresponding to the fourth pattern  114 . The first to fourth pattern images  111   p  to  114   p  may be rotationally symmetric, for example ±90°, 180°, 270°, 360° around a first image center  110   c  of the first image  110   p.    
         [0041]    The first image center  110   c  may be defined at a cross point between a horizontal line  110   x  and a vertical line  110   y.  The horizontal line  110   x  may run through a middle of the first image  110   p  along the X direction, e.g., between the first pattern image  111   p  and the third pattern image  113   p.  The vertical line  110   y  may run through a middle of the first image  110   p  along the Y direction, e.g., between the second pattern image  112   p  and the fourth pattern image  114   p.  The horizontal line  110   x  may be used to measure an overlay along the Y direction while the vertical line  110   y  may be used to measure an overlay along the X direction. 
         [0042]    Referring to  FIGS. 1 and 3B , a second image  120   p  may be acquired by selectively capturing the second zones  121  to  124  each having the second overlay marks  120  from the overlay mark  100 . The second image  120   p  may include a first pattern image  121   p  corresponding to the first pattern  121 , a second pattern image  122   p  corresponding to the second pattern  122 , a third pattern image  123   p  corresponding to the third pattern  123 , and a fourth pattern image  124   p  corresponding to the fourth pattern  124 . The first to fourth pattern images  121   p  to  124   p  may be rotationally symmetric, for example 190°, 180°, 270°, 360° around a second image center  120   c  of the second image  120   p.    
         [0043]    The second image center  120   c  may be defined at a cross point between a horizontal line  120   x  and a vertical line  120   y.  The horizontal line  120   x  may run through a middle of the second image  120   p  along the X direction, e.g., between the first pattern image  121   p  and the third pattern image  123   p.  The vertical line  120   y  may run through a middle of the first image  110   p  along the Y direction, e.g., between the second pattern image  122   p  and the fourth pattern image  124   p.  The horizontal line  120   x  may be used to measure an overlay along the Y direction while the vertical line  120   y  may be used to measure an overlay along the X direction. 
         [0044]    The first image  110   p  and the second image  120   p  may be acquired by an overlay measurement system or metrology tool  10  that can select two or more wavelengths as described, for example, in  FIG. 4 . With reference to  FIG. 4 , the overlay measurement system  10  may include a light source  11  for emitting incident light  21  towards a wafer  30 , a filter  13  for selectively passing a portion of output light  23  scattered from the wafer  30 , and a camera  15  for generating an image of the overlay mark  100  based on filtered output light  25 . The filter  13  may be configured to selectively allow particular colors to pass such as red, yellow, green, or blue. The overlay mark  100  may be provided on the wafer  30 . The first overlay marks  110  may be formed on a first layer and the second overlay marks  120  may be formed on a second layer over the first layer of the wafer  30 . An additional layer  32  may be interposed between the first and second layers of the wafer  30 . The additional layer  32  may be a single layer or multiple layers. 
         [0045]    The first overlay marks  110  may be disposed below one or more additional layers  32 . Therefore, the first image  110   p  may be acquired based on light that passes through one or more additional layers  32 . The second image  120   p  may be acquired based on light that does not pass through the additional layer  32 . There may be thickness differences between the first and second layers of the wafer  30 . Different wavelengths may be used to generate clear first and second images  110   p  and  120   p,  respectively. 
         [0046]    According to an exemplary embodiment of the present inventive concept, the first image  110   p  may be obtained by selecting and capturing the first overlay marks  110  based on light having a first wavelength. The second image  120   p  may be obtained by selecting and capturing the second overlay marks  120  based on light having a second wavelength different from the first wavelength. For example, the second wavelength may be shorter than the first wavelength. In other words, light used to obtain the first image  110   p  may have a wavelength longer than that of light used to obtain the second image  120   p.  Alternatively, the first wavelength may be shorter than the second wavelength. 
         [0047]    For example, the light used to capture the first image  110   p  may have red color in a range of visible light and the light used to capture the second image  120   p  may have blue color in the range of visible light. Alternatively, at least one of the light for capturing the first image  110   p  and the light for capturing the second image  120   p  may be from a range of visible light, a range (e.g., infrared rays) longer than that of visible light, or a range (e.g., ultraviolet rays) shorter than that of visible light. The first image  110   p  may be acquired by entirely selecting and capturing the overlay mark  100  based on light having the first wavelength, for example light having red color. In this case, clearness of an image corresponding to the first overlay marks  110  may be greater than that of an image corresponding to the second overlay marks  120 . The second image  120   p  may be acquired by entirely selecting and capturing the overlay mark  100  based on light having the second wavelength, for example light having blue color. In this case, clearness of an image corresponding to the second overlay marks  120  may be greater than that of an image corresponding to the first overlay marks  110 . 
         [0048]    Referring to  FIGS. 3C and 3D , there may be acquired a combined image  130   p  in which one of the first and second images  110   p  and  120   p  lies on top of the other. If there is zero overlay error, the first image center  110   c  may coincide with the second image center  120   c.    
         [0049]    Referring to  FIGS. 3E and 3F , if there is an overlay error, the position of the first image center  110   c  may be inconsistent with the position of the second image center  120   c.  An X-directional overlay error ΔX and a Y-directional overlay error AY may be determined by calculating a displacement between the first image center  110   c  and the second image center  120   c.    
         [0050]      FIG. 5  is a top plan view of an overlay mark according to an exemplary embodiment of the present inventive concept.  FIG. 6  is a top plan view of an overlay mark according to an exemplary embodiment of the present inventive concept. 
         [0051]    Referring to  FIG. 5 , an overlay mark  200  may include a first overlay mark  210  in a shape of an open-centered box and a second overlay mark  220  in a shape of a closed box. For example, the first overlay mark  210  may be disposed on a first layer of a wafer and the second overlay mark  220  may be disposed on a second layer over the first layer of the wafer. The first overlay mark  210  may surround the second overlay mark  220 . 
         [0052]    A first image of the first overlay mark  210  may be acquired based on light having a first wavelength, a second image of the second overlay mark  220  may be acquired based on light having a second wavelength different from the first wavelength. A combined image may be acquired by overlapping the first and second images. An overlay error may be determined by calculating displacements between lines C 1  and C 2  corresponding to opposing inner edges of the first overlay mark  210  and lines C 3  and C 4  corresponding to opposing outer edges of the second overlay mark  220 . The first wavelength may be longer or shorter than the second wavelength. 
         [0053]    For example, if a distance between the lines C 1  and C 3  is substantially identical to a distance between the lines C 2  and C 4 , an overlay error along an X direction may be zero. An overlay error along a Y direction may also be determined using the above technique. 
         [0054]    Referring to  FIG. 6 , an overlay mark  300  may include first overlay marks  310  provided on a first layer of a wafer, second overlay marks  320  provided on a second layer over the first layer, and third overlay marks  330  provided on a third layer over the second layer. The first overlay marks  310  may include a plurality of lines which extend along an X direction and are spaced apart at regular intervals along a Y direction and a plurality of lines which extend along the Y direction and are spaced apart at regular intervals along the X direction. Shapes and arrangements of the second and third overlay marks  320  and  330  may be identical or analogous to those of the first overlay marks  310 . The first to third overlay marks  310  to  330  may be horizontally spaced apart from each other and might not vertically overlap with each other. 
         [0055]    A first image corresponding to the first overlay marks  310  may be acquired based on light having a first wavelength. A second image corresponding to the second overlay marks  320  may be acquired based on light having a second wavelength. A third image corresponding to the third overlay marks  330  may be acquired based on light having a third wavelength. A combined image may be acquired by overlapping at least two of the first to third images. An overlay error may be determined by calculating displacements between at least two image centers of the overlapped two images. The first to third wavelengths may be different. For example, the second wavelength may be shorter than the first wavelength, and the third wavelength may be shorter than the second wavelength. Alternatively, the first wavelength may be shorter than the second wavelength, and the second wavelength may be shorter than the third wavelength. The second wavelength may be shorter than the first and third wavelengths, and one of the first and third wavelengths may be longer or shorter than the other. 
         [0056]    According to exemplary embodiments of the present inventive concept, variation of light wavelength depending on the position of the overlay mark may increase image clarity. Accordingly, reliable determinations of overlay errors may increase yield and improve electrical characteristics of semiconductor devices. Moreover, overlay errors may be monitored in real time and the occurrence of manufacturing errors may be reduced. 
         [0057]    While the inventive concept has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the present inventive concept as defined by the following claims.