Patent Publication Number: US-6984531-B2

Title: Electrical field alignment vernier

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application is a divisional of application Ser. No. 10/114,707, filed on Apr. 1, 2002 now U.S. Pat. No. 6,762,432, entitled “Electrical Field Alignment Vernier” of the same inventor hereof, which application is incorporated herein by reference in its entirety. 

   FIELD OF THE INVENTION 
   This invention relates to test structure patterns used in semiconductor manufacturing, and in particular to test structure patterns used to determine the field-to-field alignment of a stepper in a lithographic process. 
   DESCRIPTION OF RELATED ART 
   Photomasks are an integral component in the lithographic process of semiconductor manufacturing. Semiconductor manufacturers use photomasks to optically transfer (e.g., print) images of devices (e.g., integrated circuits) onto semiconductor wafers. A lithography tool called stepper projects light through the photomask to print the image of one or more devices onto a field on a silicon wafer coated with photoresist. The stepper then moves (e.g., steps) the wafer and the image is exposed once again onto another field on the wafer. This process is repeated for the entire wafer surface. When using a positive photoresist, the exposed portions of the photoresist are removed so areas of the wafer underneath can either be etched to form channels or be deposited with other materials. This process can be reversed using a negative photoresist where the unexposed portions of the photoresist are removed. 
     FIG. 1  illustrates a path  102  of a stepper on a wafer  100  coated with photoresist. The stepper prints the image of one or more devices on fields  200 - 1 ,  200 - 2  . . .  200 - j  . . .  200 - m  on wafer  100 , where “j” and “m” are variables. 
     FIG. 2  illustrates that each field partially overlaps neighboring fields in scribe lanes (also called “scribe streets” or “scribe lines”) where a dicing tool cuts to separate the fields. For example, the left edge of field  200 - 1  and the right edge of field  200 - 2  overlap in scribe lanes  202  and  210 , the lower edge of field  200 - 1  and the upper edge of field  200 - 7  overlap in scribe lanes  208  and  210 , and the lower left corner of field  200 - 1  and the upper right corner of field  200 - 6  overlap in scribe lane  210 . Similarly, the upper edge of field  200 - 6  and the lower edge of field  200 - 2  overlap in scribe lanes  204  and  210 , and the right edge of field  200 - 6  and the left edge of field  200 - 7  overlap in scribe lanes  206  and  210 . 
   In lithography, field-to-field alignment of the stepper is critical because it impacts all future masking alignments, wafer sort, and ultimately the assembly process. If the field alignment is poor, it directly impacts sort yield and assembly yield. The assembly process can be halted if the field-to-field alignment is so poor that the dicing tool cuts into the production die and damages the die and itself. By quantifying the amount of misalignment, steppers that need maintenance may be detected before they damage or destroy product wafers. 
   Thus, what is needed is a production friendly, field-to-field alignment tool that allows the ability to rapidly and accurately measure and quantify the field-to-field alignment. 
   SUMMARY OF THE INVENTION 
   In one embodiment of the invention, a test structure pattern includes a first comb having a first set of tines, and a second comb having a second set of tines of the same width and spacing as the first set of tines. When the test structure pattern is stepped between fields on a wafer, the first comb and the second comb at least partially overlap on photoresist over a scribe lane between the fields. When the photoresist is developed, the overlap of the first comb and the second comb generates a metal comb. Electrical continuity is checked for the metal tines of the metal comb to determine the amount of misalignment of the fields. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a conventional path of a stepper in photolithography. 
       FIG. 2  illustrates a number of conventional fields with overlapping scribe lane so n a wafer. 
       FIG. 3  illustrates a top view of test structure patterns on a photomask used with positive photoresist in accordance with one embodiment of the invention. 
       FIG. 4  illustrates an enlarged view of a receive comb of  FIG. 3  in one embodiment of the invention. 
       FIG. 5  illustrates an enlarge view of a send comb of  FIG. 3  in one embodiment of the invention. 
       FIG. 6  illustrates a top view of the overlap of the send comb and received comb after the photomask is stepped through neighboring fields in one embodiment of the invention. 
       FIG. 7  illustrates a test structure generated on the photoresist form exposure to the send comb and the receive comb with an aligned stepper in one embodiment of the invention. 
       FIG. 8  illustrates a top view of the non-overlap of the send comb and received comb after the photomask is stepped through neighboring fields in one embodiment of the invention. 
       FIG. 9  illustrates a test structure generated on the photoresist form exposure to the send comb and the receive comb with a misaligned stepper in one embodiment of the invention. 
   

   DETAILED DESCRIPTION 
     FIG. 3  illustrates a photomask  300  for use with positive photoresist in one embodiment of the invention. Photomask  300  includes two sets of test structure patterns. Each set of test structure patterns includes a send comb  302 - i  and a receive comb  304 - i , where “i” is a variable. Send comb  302 - i  and receive comb  304 - i  are mirrored in opposing scribe lanes  306 - i  and  308 - i . Specifically, (1) a first set of test structure patterns includes (a) an opaque send comb  302 - 1  on a left scribe  306 - 1  and (b) an opaque receive comb  304 - 1  on a right scribe  308 - 1 , and (2) a second set of test structure patterns includes (a) an opaque send comb  302 - 2  on a lower scribe  306 - 2  and (b) an opaque receive comb  304 - 2  on an upper scribe  308 - 2 . 
     FIG. 4  illustrates a receive comb  304 - i  in one embodiment. Dashed lines  410  and  412  delineate the respective outward and inward boundaries of scribe  308 - i . Receive comb  308 - i  includes parallel tines  402 - 1 ,  402 - 2 ,  402 - 3 ,  402 - 4 ,  402 - 5 , . . . , and  402 - n , where “n” is a variable (collectively as “tines  402 ”). In one embodiment, send comb  304 - i  includes fifteen (15) tines  402  of different widths that increment from 1 micron to 6 microns in 0.5 micron steps, and then from 7 to 10 microns in 1 micron steps. Tines  402 - 1  to  402 - 11  are spaced apart by 6 microns while tines  402 - 11  to  402 - 15  are spaced apart by 10 microns. In other embodiments, tines  402  could be of other widths and spacing to detect a specific misalignment range. Tines  402  are coupled to a common line  404  that runs across the inward ends of tines  402 . A line  406  couples line  404  to a single bond pad  408 . 
     FIG. 5  illustrates a send comb  302 - i  in one embodiment. Dashed lines  510  and  512  delineate the respective inward and outward boundaries of scribe  306 - i . Similar to receive comb  304 - i , send comb  302 - i  includes parallel tines  502 - 1 ,  502 - 2 ,  502 - 3 ,  502 - 4 , . . . , and  502 - n  (collectively as “tines  502 ”). In one embodiment, tines  402  and  502  have the same number and widths. Tines  502  are individually coupled to respective lines  506 - 1 ,  506 - 2 ,  506 - 3 ,  506 - 4 ,  506 - 5 , . . . , and  506 - n  (collectively as “lines  506 ”). Lines  506  are individually coupled to bond pads  508 - 1 ,  508 - 2 ,  508 - 3 ,  508 - 4 ,  508 - 5 , . . . , and  508 - n  (collectively as “bond pads  508 ”) so tines  502  may be individually probed. In one embodiment, each of lines  506  has at least the same width as the tine that it is coupled to. To maintain a mirror image of receive comb  304 - i , send comb  302 - i  also includes a common line  504  that runs across the outward ends of tines  502 . 
   In embodiments of the invention, photoresist is formed atop a conductive layer on a wafer. As described above, opposing scribes overlap on a scribe lane between fields on the wafer when a stepper moves photomask  300  between the fields.  FIG. 6  illustrates that tines  502  and line  504  of send comb  302 - i  at least partially overlap respective tines  402  and line  404  of receive comb  304 - i  (or vice versa) in the scribe lane if the stepper has accurately placed and aligned the fields. In  FIG. 6 , send comb  302 - i  and receive comb  304 - i  are shaded by lines of two different orientations while the overlap of send comb  302 - i  and receive comb  304 - i  are shaded by lines of both orientations. Thus, areas of the photoresist under (1) the overlap of tines  402  and  502  and (2) the overlap of lines  404  and  504  are left unexposed. 
     FIG. 7  illustrates the resulting structure from the pattern of  FIG. 6  when the photoresist is developed and the conductive layer is etched. The overlap of tines  402  and  502 , and the overlap of lines  404  and  504 , form a metal comb  702  in the scribe lane. Metal comb  702  includes unbroken metal tines  704 - 1  to  704 - n  (collectively as “metal tines  704 ”). Line  406  and bond pad  408  respectively form a metal line  706  and a bond pad  708  on a first of two adjacent fields. Lines  506  and bond pads  508  respectively form metal lines  716 - 1  to  716 - n  and metal bond pads  718 - 1  to  718 - n  (collectively as “metal bond pads  718 ”) on a second of two adjacent fields. When a current is supplied to each of metal bond pads  718 , continuity is established at metal bond pad  708  because each of metal tines  704  is unbroken. 
     FIG. 8  illustrates that some of tines  502  and line  504  of send comb  302 - i  do not partially overlap the corresponding tines  402  and line  404  of receive combs  304 - i  in the scribe lane (or vice versa) if the stepper has not accurately placed and aligned the fields. In  FIG. 8 , send comb  302 - i  and receive comb  304 - i  are shaded by lines of two different orientations while the overlap of send comb  302 - i  and receive comb  304 - i  are shaded by lines of both orientations. The misalignment of send comb  302 - i  and receive combs  304 - i  causes light to land on the non-overlapping areas masked only by send comb  302 - i  or receive comb  304 - i . As previously described, areas of photoresist under (1) the overlap of tines  402  and  502  and (2) the overlap of lines  404  and  504  are unexposed. However, the non-overlapping areas will have the conducting layer underneath etched off. 
     FIG. 9  illustrates the resulting structure from the pattern of  FIG. 8  when the photoresist is developed and the conductive layer is etched. The overlap of tines  402  and  502 , and the overlap of lines  404  and  504 , form a metal comb  902  in the scribe lane. Metal comb  902  includes broken or missing metal tines  904 - 1  and  904 - 2  (shown with dashed lines), and unbroken metal tines  904 - 3  to  904 - n . Line  406  and bond pad  408  respectively form a metal line  906  and a metal bond pad  908  on a first of two adjacent fields. Lines  506  and bond pads  508  respectively form metal lines  916 - 1  to  916 - n  and metal bond pads  918 - 1  to  918 - n  (collectively as “metal bond pads  918 ”) on a second of two adjacent fields. When a current is supplied to one of the metal bond pads  918 , continuity is not established at metal bond pad  908  if a corresponding metal tine is broken. 
   A probe card can be used to probe the resulting metal combs and record electrical continuity for each tine. The width of the smallest tines of send comb  302 - i  and receive comb  304 - i  that generate a metal tine at which electrical continuity is recorded represents the largest amount of misalignment for that field in a direction perpendicular to the tines. For example, the fields of  FIG. 9  are aligned within a dimension equal to the width of tine  402 - 3  of send comb  302 - i  and tine  502 - 3  of receive comb  304 - i.    
   Various other adaptations and combinations of features of the embodiments disclosed are within the scope of the invention. As understood by one skilled in the art, the concepts discussed herein can be implemented with a photomask used with negative photoresist where the transmission characteristics are reversed. Numerous embodiments are encompassed by the following claims.