Abstract:
The present invention is a system and method for use with alignment marks and search algorithms of diffraction pattern detection tools. The system and method of the invention significantly increases the capture range of diffraction pattern detection methods and enable more efficient operation of tools employing such detection methods.

Description:
BACKGROUND OF INVENTION 
   1. Field of the Invention 
   The present invention is a system and method for using alignment marks on a substrate for alignment with a photolithographic system. 
   2. Background of the Invention 
   Alignment tools, such as photolithographic systems, typically do a global search to provide a coarse alignment of a substrate that is loaded into the system. This coarse alignment locates alignment marks within a specified tolerance and removes large translation and rotation errors in the placement of the substrate in the system before a finer grained alignment is performed for the substrate. Removal of these potentially large errors during coarse alignment enables fine alignment systems to efficiently capture alignment marks on the substrate during the fine alignment process. 
   Fine alignment systems generally use one of three types of detection methods: bright field, dark field and diffraction pattern. Of these methods, diffraction pattern detection tends to have the smallest capture range during fine alignment. This small capture range is due to the repetition of signal peaks, which are used to identify fine alignment position, in the diffraction of an alignment source signal incident on the diffraction pattern of an alignment mark. The diffraction pattern signal received at a signal detector of the lithographic system has multiple peaks, as shown in  FIG. 1A  to  FIG. 1C . These multiple peaks can cause confusion in determining the proper aligned position of the substrate. The confusion is due to the fact that the multiple peaks appear identical to the alignment system. The capture range of the primary diffracted signal is equal to +/−½ the period of the diffraction pattern signal. Thus, exceeding this capture range during a search for the proper aligned position of the substrate can lead to large errors in substrate placement and undermine the proper operation of a fine alignment system. 
     FIG. 1A  to  FIG. 1C  show examples of background art diffraction pattern signals used during coarse alignment.  FIG. 1A  shows a diffraction pattern signal with a periodicity of 4 μm. This periodicity provides a capture range of +/−2_μμμm for the coarse alignment of a substrate.  FIG. 1B  shows a diffraction pattern signal with a periodicity of 5 μm. This periodicity provides a capture range of +/−2.5_μm for the coarse alignment of a substrate. The capture range can be extended by using a diffraction pattern signal that is the sum of diffraction pattern signals, as shown in  FIG. 1C . The periodicity of the combined diffraction pattern signals provides an extended capture range of +/−10 μm, as shown in  FIG. 1C . Though the approach of  FIG. 1C  does extend the coarse capture range, further improvement in the performance of alignment systems could be provided by even larger increases in the coarse capture range. 
   In addition, background art diffraction detection systems use either: (1) a completely separate alignment system that locates the fine alignment marks to less than +/− the fine alignment capture range; or (2) a diffraction alignment system that determines the fine alignment to allowable tolerances using two diffraction gratings of different frequencies. However, the need exists for an alignment technique that increases the coarse capture range and still properly aligns the substrate. Therefore, there is a need for improved coarse capture range in the art of alignment systems. 
   SUMMARY OF INVENTION 
   The present invention is a system and method for use with alignment marks and search algorithms of diffraction pattern detection tools. In particular, the system and method of the invention significantly increases the capture range of diffraction pattern detection methods and enables more efficient operation of tools employing such detection methods. 
   The present invention is a system of alignment marks formed on a substrate, the substrate to be used in a photolithographic system having first and second alignment signal sources and signal detectors, the alignment marks comprising: a first region configured to provide a first signal to the first signal detector in response to the first alignment signal source; and a second region configured to provide a second signal to the second signal detector in response to the second alignment signal source, wherein the first signal determines multiple first coordinates an aligned position of the substrate, the second signal determines multiple second coordinates of an aligned position of the substrate, and the substrate is in a coarse aligned position with the photolithographic system when one of the multiple first coordinates and one of the multiple second coordinates correspond to a maximum received signal strength at the first signal detector and the second signal detector, respectively. 
   Another embodiment of the present invention is a method for searching and aligning alignment marks formed on a substrate in a diffraction pattern alignment system, comprising: locating a segment of a Y mark in an X mark; illuminating the segment of a Y mark in an X mark with a Y-alignment signal source; measuring received signal strength of the diffraction pattern at a first signal detector; moving the X mark in an X-direction; repeating the illuminating, measuring and moving until the received signal strength of the diffraction pattern at the first signal detector is zero; determining a location of an approximate center of the segment of a Y mark in an X mark as a maximum of the measured received signal strength; illuminating the X mark with the X-alignment signal source; detecting multiple aligned positions received at a second signal detector as a result of illuminating the X mark; searching the multiple aligned positions detected for a single aligned position in the X-direction that corresponds to the location of the approximate center of the segment of a Y mark in an X mark; and selecting an aligned position of the X mark in the X-direction in accordance with the single aligned position that corresponds to the location of the approximate center of a Y mark in an X mark. 
   Yet another embodiment of the present invention is a method for searching and aligning alignment marks formed on a substrate in a diffraction pattern alignment system, comprising: locating a segment of an X mark in a Y mark; illuminating the segment of an X mark in a Y mark with an X-alignment signal source; measuring received signal strength of the diffraction pattern at a first signal detector; moving the Y mark in a Y-direction; repeating the illuminating, measuring and moving until the received signal strength of the diffraction pattern at the first signal detector is zero; determining a location of an approximate center of the segment of an X mark in a Y mark as a maximum of the measured received signal strength; illuminating the Y mark with the Y alignment signal source; detecting multiple aligned positions received at a second signal detector as a result of illuminating the Y mark; searching the multiple aligned positions detected for a single aligned position in the Y-direction that corresponds to the location of the approximate center of the segment of an X mark in a Y mark; and selecting an aligned position of the Y mark in the Y-direction in accordance with the single aligned position that corresponds to the location of the approximate center of an X mark in a Y mark. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1A  is an example of a diffraction pattern signal of 4 μ periodicity in the background art; 
       FIG. 1B  is an example of a diffraction pattern signal of 5 μ periodicity in the background art; 
       FIG. 1C  is an example of a diffraction pattern signal that is the sum of 4 μ and 5 μ periodicity in the background art; 
       FIG. 2  is an example of an alignment mark in the background art; 
       FIG. 3  shows a first embodiment of the alignment mark of the present invention; 
       FIG. 4  shows a second embodiment of the alignment mark of the present invention; 
       FIG. 5  shows a third embodiment of the alignment mark of the present invention; 
       FIG. 6  shows an exemplary embodiment of the alignment mark system of the present invention on a substrate; 
       FIG. 7A  shows an embodiment of an X mark with a segment of a Y mark of width W; 
       FIG. 7B  shows an alignment source of width Z; 
       FIG. 7C  shows the received signal strength at a signal detector as a result of multiple scans of an X mark with a segment of a Y mark of width W by an alignment source of width W; 
       FIG. 8  is a flow diagram for a method for aligning an X mark in the X-direction; and 
       FIG. 9  is a flow diagram for a method for aligning an Y mark in the Y-direction. 
   

   DETAILED DESCRIPTION 
     FIG. 2  shows an example of alignment marks  201 ,  203  used in the background art of alignment systems.  FIG. 3  shows a first embodiment of the alignment mark of the present invention. The embodiment shown in  FIG. 3  is an alignment mark comprising an X mark  301 ,  303  with a segment of a Y mark  302  in the X mark  301 ,  303 . In the non-limiting example shown in  FIG. 3 , the segment of a Y mark  302  in the X mark  301 ,  303  is located in the middle portion of the X mark  301 ,  303 . Moreover, a Y mark further comprising a segment of an X mark in the Y mark can also be represented by  FIG. 3  when the figure is rotated by +/−90°. 
     FIG. 4  shows a second embodiment of the alignment mark of the present invention. The embodiment shown in  FIG. 4  is an alignment mark comprising an X mark  402 ,  404 , wherein the segment of a Y mark  401 ,  405  in an X mark  402 ,  404  are located at each end of the X mark. Moreover, as discussed above for  FIG. 3 , a Y mark further comprising a segment of an X mark in the Y mark can also be represented by  FIG. 4  when the figure is rotated by +/−90°. 
     FIG. 5  shows a third embodiment of the alignment mark of the present invention. The embodiment shown in  FIG. 5  is an alignment mark comprising an X mark  501 ,  503 ,  505 ,  507 ,  509 ,  511 , wherein at least one segment or line of the X mark  502 ,  504 ,  508 ,  510  is removed. In general, segments/lines of either an X mark or a Y mark may be removed to eliminate regions of the X mark and Y mark that cause false alignments. In addition, a method for designing the alignment mark may further comprise removing a segment/line in the X mark that at least one of contribute the majority of the signal strength and require minimum signal strength. Further, removing a segment/line may further comprise replacing the removed segment/line with marks of an opposite axis to provide, for example, a coarse position check. 
   Moreover, at least one segment/line of an X mark and/or at least one segment/line of a Y mark may be removed and replaced by at least one segment/line with a different orientation than the X mark and/or Y mark, respectively.  FIG. 3 , as discussed above, shows an example of an X mark  301 ,  303  with a middle segment removed and replaced by a segment of a Y mark  302  that has a different orientation than the X mark  301 ,  303 . The different orientation of the segment is at least in the range of +/−45° to +/−90° relative to the orientation of the X mark and Y mark. 
     FIG. 6  shows an embodiment of the alignment mark system of the present invention on a substrate  601 . In particular, the alignment mark system of  FIG. 6  has a first region  602  configured to provide a first signal to a signal detector in response to a first alignment signal source. In addition, the alignment mark system has a second region  603  configured to provide a second signal to the signal detector in response to a second alignment signal source. The first region  602  and the second region  603  are an X mark diffraction pattern and a Y mark diffraction pattern, respectively. The first signal determines a multiple first coordinates (e.g., X-coordinates) of an aligned position of the substrate  601 . The second signal determines multiple second coordinates (e.g., Y-coordinates) of an aligned position of the substrate  601 . In the embodiment of the present invention shown in  FIG. 6 , the substrate  601  is in an aligned position with the photolithographic system when one of the first coordinates and one of the second coordinates correspond to the maximum received signal strength at the signal detectors for the first and second signals, respectively. 
     FIG. 7A  shows an embodiment of an X mark  701 ,  703  that has a segment of a Y mark  702  with a predetermined width W located in a middle portion of the X mark  701 ,  703 . A non-limiting example of the width W is 25 μm to 75 μm. 
     FIG. 7B  shows a beam  705  of an alignment signal source of width Z. A non-limiting example of a typical width Z for the beam of the alignment signal source is 50 μm. The beam  705  of the alignment signal source is used to illuminate the alignment mark  702  of  FIG. 7A . Note that alignment marks  701  and  703  are aligned at 90° relative to the beam  705  of the alignment signal source. Nonlimiting examples of alignment signal sources include a laser and any broadband light source. 
     FIG. 7C  shows a non-limiting example of the received signal strength at a signal detector as a result of multiple scans of an alignment mark  701 ,  702 ,  703  by a beam  705  of an alignment signal source with width Z=W. The beam  705  may also have a width Z&gt;W or Z&lt;W. For the case of Z=W shown in  FIG. 7C , the maximum of the signal strength  706  received at the signal detector as a result of illumination by the beam  705  occurs at the approximate center position of the Y mark segment of width W. This approximate center position is used in the method described below to determine the coordinates for aligned position of the substrate and to extend the capture range. 
     FIG. 8  is a flow diagram for a method for aligning an X mark in the X-direction. Step  801  of  FIG. 8 , involves locating a segment of a Y mark in an X mark. Next, step  802  involves illuminating the segment of a Y mark in an X mark with a Y-alignment signal source. In step  803 , measuring the received signal strength of the diffraction pattern at a signal detector is performed. During step  804 , it is determined whether or not the measured received signal strength is zero. If the measured received signal strength is not zero, (1) the X mark is moved in an X-direction in step  805 ; and (2) step  802  and step  803  are repeated. If the measured received signal strength is zero, the approximate center of the Y mark in an X mark is determined as the maximum of the received signal strength in step  806 . Further, in step  805  of  FIG. 8 , moving the segment of the X mark in an X-direction is performed by moving the X mark in relation to the Y-alignment signal source by a specified amount in the X-direction until the Y-alignment signal source is past the segment of a Y mark in an X mark. 
   Next step  807  of  FIG. 8  involves illuminating the X mark with an X-alignment signal source. Step  808  involves detecting multiple aligned positions received at the signal detector as a result of illuminating the X mark in step  807 . In step  809 , the multiple aligned positions detected in step  808  are searched for a single aligned position in the Y-direction that corresponds to the location of the approximate center of the segment of a Y mark in an X mark. Step  810  involves selecting an aligned position of the X mark in the X-direction in accordance with the single aligned position that corresponds to the location of the approximate center of a Y mark in an X mark. In addition, selecting an aligned position for the alignment mark may also comprise determining whether the received signal strength is greater than or equal to a predetermined threshold. After aligning the X mark in the X-direction, aligning the Y mark in the Y-direction is performed as discussed below. 
     FIG. 9  is a flow diagram for a method for aligning a Y mark in the Y-direction. Step  901  of  FIG. 9 , involves locating a segment of an X mark in a Y mark. Next, step  902  involves illuminating the segment of an X mark in a Y mark with an X-alignment signal source. In step  903 , measuring the received signal strength of the diffraction pattern at a signal detector is performed. During step  904 , it is determined whether or not the measured received signal strength is zero. If the measured received signal strength is not zero, (1) the Y mark is moved in a Y-direction in step  905 ; and (2) step  902  and step  903  are repeated. If the measured received signal strength is zero, the approximate center of the X mark in a Y mark is determined as the maximum of the received signal strength in step  906 . Further, in step  905  of  FIG. 9 , moving the segment of the Y mark in a Y-direction is performed by moving the Y mark in relation to the X-alignment signal source by a specified amount in the Y-direction until the X-alignment signal source is past the segment of an X mark in a Y mark. 
   Next step  907  of  FIG. 9  involves illuminating the Y mark with a Y-alignment signal source. Step  908  involves detecting multiple aligned positions received at the signal detector as a result of illuminating the Y mark in step  907 . In step  909 , the multiple aligned positions detected in step  908  are searched for a single aligned position in the X-direction that corresponds to the location of the approximate center of the segment of an X mark in a Y mark. Step  910  involves selecting an aligned position of the Y mark in the Y-direction in accordance with the single aligned position that corresponds to the location of the approximate center of an X mark in a Y mark. In addition, selecting an aligned position for the alignment mark may also comprise determining whether the received signal strength is greater than or equal to a predetermined threshold. 
   The foregoing description illustrates and describes the present invention. Additionally, the disclosure shows and describes only the preferred embodiments of the invention, but, as mentioned above, it is to be understood that the invention is capable of use in various other combinations, modifications, and environments and is capable of changes or modifications within the scope of the inventive concept as expressed herein, commensurate with the above teachings and/or the skill or knowledge of the relevant art. The embodiments described hereinabove are further intended to explain best modes known of practicing the invention and to enable others skilled in the art to utilize the invention in such, or other, embodiments and with the various modifications required by the particular applications or uses of the invention. Accordingly, the description is not intended to limit the invention to the form or application disclosed herein. Also, it is intended that the appended claims be construed to include alternative embodiments.