Patent Publication Number: US-8994197-B2

Title: Alignment mark and method of manufacturing the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional application of U.S. patent application Ser. No. 13/333,272, filed Dec. 21, 2011. 
    
    
     DESCRIPTION 
     1. Technical Field 
     The present disclosure relates generally to an alignment mark and a method of manufacturing the same, and relates more particularly to an alignment mark used for alignment in lithographic processes and a method of manufacturing the same. 
     2. Background 
     In semiconductor fabrication processes, lithographic apparatuses are applied to form desired circuit patterns on substrates. In general, photosensitive resin is coated on a wafer. A lithographic apparatus radiates light on a reticle carrying a circuit layout similar to, but larger than, a desired circuit pattern, transferring the circuit layout to the photosensitive resin on the wafer. Known lithographic apparatuses include so-called steppers, which project light on an entire circuit layout to instantly transfer the circuit layout on a wafer in a shot at every exposure operation, and so-called scanners, which move a reticle and a wafer synchronously to allow a scanning light beam that can illuminate a specific portion of a circuit layout to gradually transfer the circuit layout on a shot area of a wafer at every exposure operation. 
     It is widely known that the resolution capability of a lithographic apparatus (i.e., the minimum feature that can be printed) can be described by the Rayleigh equation: 
     
       
         
           
             
               
                 
                   resolution 
                   = 
                   
                     
                       k 
                       1 
                     
                     ⁢ 
                     
                       λ 
                       
                         numerical 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         aperture 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     Equation (1) shows how resolution of a lithographic apparatus can be improved by reduction of the wavelength λ of exposure light. Indeed, the electronic industry has improved resolution by developing lithographic apparatuses based on successively shorter wavelengths: g-line (436 nm), i-line (365 nm), DUV (248 nm), and ArF (193 nm). Similarly, according to Equation (1), resolution can be also improved by creating lithographic apparatuses with larger numerical apertures. In addition, the resolution may be enhanced by optimization of process parameters including illumination settings, mask design and photoresist process. The process parameters can be included in the k 1  factor. 
     Depth-of-focus (DOF) is another important parameter measuring the performance of a lithographic apparatus. DOF can be improved using the immersion technique, alternating phase-shift mask, or off-axis illumination. 
     Dipole is an off-axis illumination process that provides better aerial image contrast than conventional or annular illumination can provide, in directions perpendicular to the dipole orientation. Specifically, X-dipole illumination gives the best resolution for vertical lines and Y-dipole gives the best resolution for horizontal lines. However, X-dipole does not give good resolution for horizontal lines, and Y-dipole does not give good resolution for vertical lines. 
     In order to accurately project the circuit layout on a desired position of the wafer, the wafer must be aligned before an exposure operation is performed. To align the wafer, alignment marks have to be formed on the wafer.  FIG. 1  illustrates alignment marks  11  and  12  commonly used for alignment in a pitch-doubling process, which usually adopts X-dipole illumination. The alignment marks  11  and  12  can be disposed respectively in scribe lines  13  and  14 . The alignment mark  11  may comprise a plurality of first mark elements  111  arrayed along a direction parallel to the x-axis and a plurality of second mark elements  112  also arrayed along a direction parallel to the x-axis. Each second mark element  112  extends along a direction parallel to the y-axis. The major bar elements  111  are formed among the plurality of second mark elements  112 . To measure the position of the wafer in the x-axis direction, an illumination spot  15  moves in a direction  16  to illuminate the alignment mark  11  so as to generate detecting signals that are then analyzed to determine the position of the wafer in the x-axis direction. 
     Similarly, the alignment mark  12  may comprise a plurality of first mark elements  121  arrayed along a direction parallel to the y-axis and a plurality of second mark elements  122  arrayed along a direction parallel to the x-axis. Each second mark element  122  extends along a direction parallel to the y-axis, and the major bar elements  111  are formed among the plurality of second mark elements  122 . To measure the position of the wafer in the y-axis direction, an illumination spot  17  moves in a direction  18  to illuminate the alignment mark  12  so as to generate detecting signals that are then analyzed to determine the position of the wafer in the x-axis direction. 
     In the alignment system shown in  FIG. 1 , the position in the x-axis direction can be accurately determined while the measured position in the y-axis direction needs an offset for compensation. In the alignment marks  11  and  12 , the second mark elements  112  and  122  all extend in directions parallel to the y-axis. Such an arrangement means that the second mark elements  112  extend perpendicular to the scanning direction  16  of the illumination spot  15  as shown in  FIG. 2 , while the second mark elements  122  extend parallel to the scanning direction  18  as shown in  FIG. 3 . When the illumination spot  17  scans along the alignment mark  12 , the illumination spot  17  may meet the ends of the second mark elements  122 . The ends of the second mark elements  122  form an irregular edge profile of the alignment mark  12  that may cause inaccuracy of alignment. 
     SUMMARY 
     In some embodiments, an alignment mark may comprise a plurality of mark units. Each mark unit may comprise a first element and a plurality of second elements. Each second element may comprise opposite first and second end portions. The plurality of second elements may be arranged along a direction. The first element may extend adjacent to the first end portions of the plurality of second elements and parallel to the direction of the arrangement of the plurality of second elements. 
     In some embodiments, an alignment mark may comprise a plurality of mark units. Each mark unit may comprise two first elements and two groups of second elements. The two first elements may extend parallel to each other. The two first elements may extend along a direction. The two first elements may be between the two groups of second elements. Each second element may comprise two opposite end portions. The second elements of each group may be arranged in a direction parallel to the direction of the extension of the two first elements. One of the two opposite end portions may extend adjacent to a corresponding one of the two first elements. 
     In some embodiments, a method of manufacturing an alignment mark may comprise forming, on a substrate, a pattern comprising a connecting member and a plurality of lines extending from a side of the connecting member, forming spacers on side walls of the pattern, removing the pattern, and etching the substrate using the spacers to form a mark unit of the alignment mark. 
     The foregoing has outlined rather broadly the features of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features of the invention will be described hereinafter, and form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures or processes for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates commonly used alignment marks; 
         FIG. 2  shows a portion of one alignment mark of  FIG. 1 ; 
         FIG. 3  shows a portion of another alignment mark of  FIG. 1 ; 
         FIG. 4  is a view showing an alignment mark according to one embodiment of the present invention; 
         FIGS. 5 to 7  are perspective views employed for demonstrating the steps of a method of manufacturing an alignment mark according to one embodiment of the present invention; and 
         FIG. 8  is a top view showing a mark unit of an alignment mark according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 4  is a view showing an alignment mark  4  according to one embodiment of the present invention. As shown in  FIG. 4 , the alignment mark  4  may be used with, but is not limited to, lithographic apparatuses employing off-axis illuminations such as X-dipole illumination, Y-dipole illumination, and a quadrupole illumination. The alignment mark  4  may comprise a plurality of mark units  41  arranged along a direction parallel to a direction  42  that can be, for example, a mark scanning direction. When an illumination spot scans along the alignment mark  4 , signals for determining the position of a wafer can be generated. 
     Each mark unit  41  may comprise a first element  411  and a plurality of second elements  412 . The first element  411  can be disposed adjacent to the plurality of second elements  412 , and may be separate from the plurality of second elements  412 . In some embodiments, the first element  411  may be disposed adjacent to the side of the plurality of second elements  412  being perpendicular to the scanning path and exhibiting an irregular profile. 
     The plurality of second elements  412  may be arranged along a direction parallel to a direction  43 , which may be perpendicular to the scanning direction  42 . In some embodiments, the plurality of second elements  412  can be spaced apart by a consistent distance d. In some embodiments, the second element  412  may comprise a width w, which can be equivalent to the distance d. In some embodiments, the plurality of second elements  412  may be arranged to form a grating. In some embodiments, the second element  412  may comprise a bar shape. In some embodiments, the width w of the second element  412  can be on the micro-scale level. In some embodiments, the width w of the second element  412  can be on the nano-scale level. 
     Each second element  412  may comprise a first end portion  4121  and a second end portion  4122 . The first end portion  4121  may be disposed opposite to the second end portion  4122 . The plurality of second elements  412  may extend along a direction parallel to the scanning direction  42  such that the first end portions  4121  and the second end portions  4122  of the second elements  412  may be arranged perpendicular to the scanning path. 
     The first element  411  may extend along a direction parallel to the direction  43 . The first element  411  may be formed adjacent to the first end portions  4121  of the plurality of second elements  412 , in front of the first end portions  4121  relative to the scanning direction  42 . As such, when an illumination spot passes the first end portions  4121  of the plurality of second elements  412 , the first element  411  may reduce the adverse influence of the first end portions  4121  of the plurality of second elements  412  on alignment signals, generating better alignment results. In some embodiments, the first element  411  may have a bar-like shape. In some embodiments, the first element  411  may be C-shaped. 
     In some embodiments, the alignment mark  4  can be employed with a lithographic apparatus adopting X-dipole illumination, and the direction  42  may be the y-axis direction. In some embodiments, the alignment mark  4  can be employed with a lithographic apparatus adopting Y-dipole illumination, and the direction  42  may be the x-axis direction. 
     In some embodiments, the plurality of second elements  412  can be separated from each other. In some embodiments, in each mark unit  41 , the plurality of second elements  412  can be arranged into a plurality of pairs  42 , and the second elements  412  of each pair  42  are connected at their first end portions  4121 . The connected first end portions  4121  of the second elements  412  of each pair  4  can further reduce the adverse influence of the first end portions  4121  of the plurality of second elements  412  on alignment signals, resulting in better alignment. 
     In some embodiments, the mark unit  41  may further comprise two third elements  413 , between which the plurality of second elements  412  are disposed. The two third elements  413  may be formed at two opposite sides of the mark unit  41 , configured to define the mark unit  41 . 
       FIGS. 5 to 7  are perspective views employed for demonstrating the steps of a method of manufacturing an alignment mark  4  according to one embodiment of the present invention. For simplification,  FIGS. 5 to 7  show the manufacturing process for only one mark unit  41  of an alignment mark  4 . The same manufacturing process can be applied to manufacture a complete alignment mark  4 . As shown in  FIG. 5 , a pattern  51  is formed on a substrate  52 . In some embodiments, the pattern  51  is formed on a scribe line. In some embodiments, the pattern  51  is formed on a location other than a scribe line. In some embodiments, the pattern  51  may comprise photoresist. In some embodiments, the pattern  51  can be formed by a photolithographic process. 
     The pattern  51  may comprise a connecting member  511  and a plurality of lines  512 . The connecting member  511  has a side from which the plurality of lines  512  extend, as shown in  FIG. 5 . The plurality of lines  512  may be equally spaced. In some embodiments, the ratio of a width w1 of lines  512  and a width s of the spaces between the lines  512  is approximately 1:3. In some embodiments, the width w2 of the connecting member  511  is greater than the width w1 of lines  512 . 
     Referring to  FIG. 6 , spacers  61  are formed on the side walls of the pattern  51 . Spacer material can be deposited using, for example, an atomic layer deposition process. Spacer material can be deposited using other deposition methods. Spacer material may comprise oxide. The spacers  61  are formed by depositing spacer material onto the pattern  51 , and a material removal process such as etching is then applied to remove the spacer material on the horizontal surfaces, leaving the spacer material on side walls of the pattern  51 . In some embodiments, the spacer material can be a hard mask material. 
     Referring to  FIGS. 6 and 7 , after the spacers  61  are formed, the pattern  51  is removed. Next, the image of the spacers  61  is transferred to the substrate  52  by an etch process. The alignment mark  4  is then completed. 
       FIG. 8  is a top view showing a mark unit  81  of an alignment mark according to another embodiment of the present invention. The alignment mark may comprise one or more mark units  81 . Referring to  FIG. 8 , the mark unit  81  may comprise two first elements  811  and two groups  82  of second elements  821 . The two first elements  811  may extend parallel to each other. The two first elements  811  may extend along a direction  83 . The two first elements  811  may be disposed between the two groups  82  of second elements  821 . 
     In each group  82 , the second elements  821  may be arrayed along a direction parallel to the direction  83 . Each second element  821  may extend along a direction  84  that may be perpendicular to the direction  83 . The second elements  821  may have similar widths. The second elements  821  may be spaced equally by a distance that may be substantially equivalent to the width of the second element  821 . In some embodiments, the second elements  821  may form a grating. In some embodiments, the second elements  821  may be separate. 
     Each second element  821  may comprise two opposite end portions  8211  and  8212 . The second elements  821  of each group  82  can be separate from each other, and can be arranged along the direction  83 . Each first element  811  may be disposed adjacent to the end portions  8211  of the corresponding second element  821  that are arranged perpendicular to an alignment scanning path so that the adverse influence of the end portions  8211  of the second elements  821  on alignment signals can be reduced. 
     As shown in  FIG. 8 , the second elements  821  of each group  82  may be arranged into a plurality of pairs  85 . The end portions  8211  of the second elements  821  of each pair  85 , adjacent to the corresponding first element  811 , can be connected such that the adverse influence of the end portions  8211  of the second elements  821  on alignment signals can be further reduced. 
     In some embodiments, the mark unit  81  may comprise a frame member  86 , which can be between two groups  82  of second elements  821  and can comprise the two first elements  811 . In some embodiments, the width of the frame member may be equivalent to the width of the second member  821 . 
     As shown in  FIG. 8 , the mark unit  81  may further comprise two third elements  87 . The groups  82  of second elements  821  may be between the two third elements  87 . The two third elements  87  can be located at two opposite sides of the mark unit  81 , defining the mark unit  81 . 
     Although the present invention and its objectives have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. For example, many of the processes discussed above can be implemented in different methodologies and replaced by other processes, or a combination thereof. 
     Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.