Abstract:
A method is provided for using a point of interest as a starting point where an alignment is automatically selected by recognition software for a patterned substrate. The method includes disposing the patterned substrate on a stage of an exposure system, the exposure system having an alignment routine including; locating a first point of interest on the patterned substrate; scanning a first area proximate the first point of interest for a first unique feature; defining a periodicity for the patterned substrate; locating a second point of interest based on the periodicity; scanning a second area proximate the second point of interest for a second unique feature corresponding to the first unique feature; gathering alignment data from at least scanning the first and second areas; and determining substrate position relative to the exposure system from alignment data of at least the first and second scanned areas.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of U.S. application Ser. No. 10/604,010, filed Jun. 20, 2003, now U.S. Pat. No. 7,375,809, the contents of which are incorporated herein in their entirety. 

   BACKGROUND OF INVENTION 
   This disclosure relates to optical alignment systems and more particularly, to a system and method for improving an alignment routine for lithography or pattern recognition. 
   Integrated circuit chips are fabricated one level at a time. The levels include diffusions, gates, metal lines, insulation, isolation, and contacts. The structures on these levels must be precisely positioned so that the finished chip has structures properly positioned. The step of positioning a level with respect to a previously formed level is called alignment. 
   Current industry methodologies require alignments of some feature to allow for orientation of a part. More specifically, alignment of patterned materials involves manual selection of alignment targets. The alignment targets typically include two or three alignment marks which a recognition system uses to learn a position of the part or material subject to examination. The user then provides a region of interest (ROI) by manually moving to this location while the software records this coordinate. Alternatively, a coordinate relative to some known reference point (e.g., center of the part) is indicated by the software. 
   Furthermore, if multiple patterns exist on the part as found in semiconductor wafers, then a step periodicity is supplied to find the next ROI. Once the setup is completed (i.e., after the alignment marks are recorded) the optical system aligns and moves to a ROI on the part for measurement or inspection. 
   One drawback to the above approach is that time is needed to teach the alignment mark locations relative to a ROI and is wasted time. Furthermore, pattern recognition systems which fail to align are unable to re-teach themselves since no point of origin has been established. 
   SUMMARY OF INVENTION 
   In an exemplary embodiment, a method of aligning a patterned substrate having a plurality of segments and measuring the same includes defining a point of interest for each segment of the patterned substrate; locating a first point of interest in a first segment of the patterned substrate, the first point of interest being within a single die of a semiconductor wafer; scanning a first area of the patterned substrate proximate the first point of interest for a first unique feature within the single die; saving a scanned image of the first area; defining a periodicity for the patterned substrate, the periodicity corresponding to the scan area of a raster movement used in locating the first unique feature; locating a second point of interest in a second segment of the patterned substrate; scanning a second area of the patterned substrate proximate the second point of interest, based on the defined periodicity used in locating the first unique feature, for a second unique feature corresponding to the first unique feature, wherein the first unique feature is saved as an alignment image for use in locating the second unique feature in the second area; mapping the alignment of the substrate with respect to tooling in which it is disposed with; measuring the second point of interest; and locating a third point of interest in a third segment based on the periodicity. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1   a  is a side view of a photolithography system having an alignment system; 
       FIG. 1   b  is a more detailed view of the alignment system of  FIG. 1   a;    
       FIG. 2  is a top view of a wafer having multiple exposure fields and an alignment target or unique feature in each exposure field; 
       FIG. 3  is a flow chart showing prior art process steps for alignment with respect to alignment marks and a region of interest; 
       FIG. 4  is a flow chart showing process steps for an exemplary embodiment of the present invention; 
       FIG. 5  illustrates a patterned semiconductor wafer subjected to two alignment scans; 
       FIG. 6  illustrates a portion of the wafer of  FIG. 1  being scanned at a first point of interest to locate a unique feature within the scanned area; 
       FIG. 7  illustrates a portion of the scanned area of  FIG. 6  detailing a first unique feature therein; and 
       FIG. 8  illustrates a portion of a second scanned area having a second unique feature like the first unique feature of  FIG. 7 . 
   

   DETAILED DESCRIPTION 
   Photolithography tools have alignment systems for aligning a level to be printed with a level already on the wafer. The alignment system first determines the location of an alignment target on the wafer that was printed during a previous photolithographic process step. Once the location of the alignment target is established, the system adjusts the location of the wafer so the present level is printed at the proper location with respect to that previous level target. The previous level target can be the first level printed, the last level so far printed, or any level in between. 
   Alignment systems on photolithography tools are generally designed to work with specific types of alignment marks on the wafer. The alignment system acquires an alignment signal by optically scanning an alignment mark. The alignment signal is then analyzed to determine the location of the alignment mark on the wafer. This is repeated for several alignment marks in different exposure fields on the wafer. From three to eight marks are typically used. The data from this group of alignment marks is then processed with a computer in the photolithography tool to determine the location of the wafer. 
   Commonly a photolithography tool has an alignment system capable of reading several alignment marks. Furthermore, a number of alignment signal analysis algorithms can be applied to the alignment signal. The term “alignment component” will be used in this application to describe alignment system hardware, alignment marks on a substrate, and alignment signal analysis software. 
   The present invention provides a more robust alignment routine that saves time and provides a path to auto correct the alignment routine should it fail during operation. 
   The invention applies to optically based tools for inspection and/or measurement, such as for example, step-and-repeat or step-and-scan photolithography tools such as tool  20  shown in  FIG. 1   a . Photolithography tool  20  includes laser interferometer  22  which controls wafer stage  24 . Tool  20  also includes an alignment system  26 , and computer  28  to analyze alignment data. A substrate, such as silicon wafer  30 , coated with a photosensitive layer  32  and containing alignment marks or unique features  34  on prior level  36  is located on wafer stage  24 . 
   In operation, alignment system  26  sequentially shines incident light beam  44  from light source  45  on alignment mark  34  on wafer  30 , as shown in more detail in the enlarged view of  FIG. 1   b.    
   In a typical alignment process as presently practiced in the industry, light beam  44  from alignment system  26  is focused on a portion of alignment mark  34  on wafer  30  and is diffracted by that portion over a wide range of angles. Some of that diffracted light  44 ′ is gathered by detector  46  which converts it to an electrical signal which is transmitted along wire  48  to signal analyzer  50  which includes an analog to digital converter. Wafer stage  24  is moved so that incident light  44  scans across alignment target  34  so signals are eventually received from all portions of alignment mark  34 . As alignment target  34  is scanned, light signal  44 ′ striking detector  46  varies in intensity, and electrical signal output  48  from detector  46  correspondingly varies. Signal  48  is correlated with wafer stage position information from interferometer  22  to produce alignment signal  54 , the output of signal analyzer  50 . The location of alignment target  34  is then determined from characteristics of electrical signal  54  using computer  28 . This signal is analyzed with standard analysis algorithms. However, parameters of these algorithms can be changed and evaluated using the method of the present invention. For example, alignment can be expedited and provide an automatic correction should the alignment routine fail during operation. Exposure system  20  also includes illuminator  56 , reticle  58 , and objective lens  60 , as shown in  FIG. 1   a.    
   In standard practice, step-and-repeat or step-and-scan photolithography tools place exposure fields  70   a ,  70   b  . . .  70   i  on wafer  30  in an array pattern, as shown in the top view of  FIG. 2 . The first exposure field  70   a  at which alignment data is to be collected is manually positioned in the field of view of alignment system  26 , and the location of alignment mark  34   a  in field  70   a  is determined as shown in block  101  of the flow chart of  FIG. 3 . The location of an alignment mark may be defined as the location of the center of the mark, and this x-y location is saved for later processing. 
   Next, second alignment field  70   b  is positioned in the field of view of alignment system  26 , and the location of alignment target  34   b  of field  70   b  is determined, as provided in standard practice, well known in the industry. This procedure is continued for additional exposure fields  70   c ,  70   d ,  70   e  or to a number of fields chosen by the user. At least two or three fields are currently required for a recognition system to learn a position of wafer  30  being subjected to examination. 
   Once the alignment marks have been recorded an alignment image is saved at block  103 , a periodicity is defined at block  105  to move between different points of interest for examination on wafer  30 . At block  107 , the system is aligned based on the saved alignment image and periodicity size and a user manually moves to a point of interest of an exposure field an offset distance at block  108  and takes measurements at block  109  before moving to the next measurement location at block  111 . 
   However, it should be noted that the when the above system fails to align at block  107 , it cannot re-teach itself since no point of reference has been established in blocks  101  and/or block  103 . 
   Referring now to  FIGS. 4-8 , an exemplary method for an alignment subroutine will now be described. A flowchart in  FIG. 4  illustrates one exemplary method generally at  200  while  FIGS. 5-8  illustrate portions of wafer  30  being scanned at a point of interest or measurement location  202 . 
   First at block  204 , measurement location or point of interest  202  is defined. As illustrated, point of interest  202  with respect to wafer  30  is within a single die  206  of wafer  30 . Next, wafer stage  24  is moved at block  208  so that point of interest  202  may be scanned to locate a unique feature  210  within a scan area  212  proximate point of interest  202  ( FIGS. 5 and 6 ). It should be noted that unique feature  210  may be an alignment mark, however, one aspect of the present invention provides for elimination of providing a designated alignment mark. Instead, a unique feature  210  proximate point of interest  202  is located. 
   Next, a periodicity is defined at block  214 . It will be appreciated by one skilled in the art that scan area  212  may be as small or large as needed to locate a unique feature proximate point of interest  202 . More specifically, software performs a raster movement around point of interest  202  until it detects a feature unique within the field of view or scan area  212  at block  216 . As illustrated in FIGS.  3  and  6 - 8 , the unique feature  210  depicted therein is the letter “E”, however, the software is configured to detect other suitable unique features within each die  206 . At block  218 , the software saves an alignment image  220  depicted in  FIG. 7 . 
   At block  220 , the alignment of wafer  30  with respect to wafer stage  24  is completed after the above described alignment routine automatically locates alignment images saved at block  218  at two or three sites to determine relative part position. 
   Next, a measurement or inspection of the point of interest  202  is completed at block  222  before moving to another point of interest  302  at block  224  based on the periodicity defined at block  214 .  FIG. 8  reflects this second point of interest  302  having the alignment image  220  saved at block  218 . 
   As before, the software performs a raster movement around point of interest  302  until it detects a feature unique within the field of view or scan area  212  as in block  216 . In particular, the unique feature  210 , or letter “E” as illustrated, for example, corresponds to the saved alignment image  220 . Next, a measurement or inspection of the point of interest  302  is completed before moving to another point of interest based on the periodicity defined at block  214 . 
   The above described embodiments allow the use of the point of interest as a starting point and have the alignments selected automatically by recognition software. In this manner, setting up alignment routines is eliminated. It also provides a path to automatically correct the alignment routine should it fail during operation, since no point of origin or reference point needs to be established first. Further, separately providing designated alignment marks and initially locating the same is avoided. 
   In summary, the above described alignment routine allows the user to first move to or locate a first point of interest and supply or define a step periodicity. The software performs a raster movement around the first point of interest until it detects a feature unique within the field of view. Once a unique feature is found and saved, the system moves over the defined periodicity to locate the same unique feature in a second point of interest thereby completing the alignment routine. This alignment routine requires at least two sites to determine part position. In this manner, alignment marks are not necessary and alignment with respect to the same is eliminated. 
   While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.