Abstract:
A method includes determining defect types and defect locations on a mask blank and storing the defect types and the defect locations on the mask blank. The method further includes generating at least one alignment mark on the mask blank and selecting a mask pattern for the mask blank based on the defect types and the defect locations. Additionally, the method includes determining a positioning of the mask pattern on the mask blank, aligning a mask pattern generator with the mask blank in accordance with the positioning using the at least one alignment mark and forming the mask pattern in the mask blank using the mask pattern generator.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method and system for pre-alignment marking and inspection, and more particularly, to a method and system for pre-alignment marking and inspection to improve mask substrate defect tolerance. 
       BACKGROUND OF THE INVENTION 
       [0002]    Photolithography (also optical lithography) is a process used in microfabrication to selectively remove parts of a thin film (or the bulk of a substrate). Photolithography uses light to transfer a geometric pattern from a photomask to a light-sensitive chemical (photoresist, or simply “resist”) on the substrate. Elements of a circuit to be created on the IC are reproduced in a pattern of transparent and opaque areas on the surface of the photomask (or reticle), e.g., a quartz plate. A stepper passes light through the photomask, forming an image of the photomask pattern. The image may be focused and reduced by a lens, and projected onto the surface of a silicon wafer that is coated with the photoresist. 
         [0003]    After exposure in the stepper, the coating on the wafer is developed like photographic film, causing the photoresist to dissolve in certain areas according to the amount of light the areas received during exposure. These areas of photoresist (and no photoresist) reproduce the pattern on the photomask. The developed wafer is then exposed to acids or other chemicals. The acids etch away the silicon in the parts of the wafer that are no longer protected by the photoresist coating. The other chemicals are used to change the electrical characteristics of the silicon in the bare areas. The wafer is then cleaned, recoated with photoresist, then passed through the stepper again, e.g., using another photomask, in a process that creates the circuit on the, e.g., silicon substrate layer by layer. The entire process is called photolithography. In a complex integrated circuit (e.g., a modern CMOS device), a wafer will go through the photolithographic cycle on the order of 50 times. 
         [0004]    The photomask is an opaque plate with topography or transparencies that allow light to shine through in a defined pattern. Lithographic photomasks may be, e.g., transparent fused silica or quartz blanks covered with a pattern defined with, e.g., a chrome metal absorbing film. In order to fabricate an integrated circuit, a set of photomasks, each defining a pattern layer in the integrated circuit fabrication, is fed into a photolithography stepper or scanner and individually selected for exposure, as described above. Thus, prior to the fabrication of the integrated circuit device using the photolithographic process, the photomasks are manufactured. 
         [0005]    In photomask manufacturing, e.g., quartz blanks are coated with an absorbing material and an imaging film (i.e., a photoresist) to form the photomask pattern. The image for the photomask may originate from a computerized data file. This data file is converted to a series of polygons and written onto, e.g., a square fused quartz substrate covered with a layer of chrome using a photolithographic process. A beam of electrons may be used to expose the photomask pattern defined in the data file and travels over the surface of the photomask blank in either a vector or raster scan manner. Where the photoresist on the photomask is exposed, the chrome can be etched away, leaving a clear path in the photomask for the light in the stepper/scanner systems to travel through. 
         [0006]    However, a relatively small number of defects on the photomask blank might manifest themselves as defects during photomask patterning, which can affect final photomask quality and result in yield loss (e.g., wafer and/or photomask yield loss) and additional defect repair work. For example, performing photolithography with a photomask that includes defects may cause defects to be formed in the integrated circuit device. Thus, defects in a photomask have a direct impact on photomask manufacturing costs and turn-around times. 
         [0007]    The preferred solution to yield loss from photomask blank defects is to lower printable defect count on the photomask blank with cleaner processing and/or repair. However, obtaining defect free photomask blanks (or lowering photomask blank defect count) is a significant challenge as lithographic feature size and the minimum printable defect size continue to be reduced. Moreover, photomask blank defects in new materials that may be needed for the AEPSM (Attenuated Embedded Phase Shift Mask) have increased. This increase in photomask blank defects has in turn reduced the yield after pattern defect inspection and, consequently, final photomask yield. Moreover, next generation lithography techniques, such as electron projection lithography (EPL) and extreme ultraviolet lithography (EUVL), require photomask blanks made from different materials than binary optical masks or AEPSM. These new photomask blanks may initially have more defects than present photomask blanks for optical projection lithography. Therefore, a method to pattern masks with high yield on mask blanks that have defects is desirable. 
         [0008]    To enable the use of mask blanks that are not defect free but have a reasonably small number of defects, the pattern on the photomask can be moved so that the photomask blank defects do not affect the printed pattern. If mask blank defects can be accurately located and evaluated, the patterning process can be modified to accommodate some defects on the blank. 
         [0009]    However, there is no ability to reliably align the photomask blank with the pattern generator to ensure a higher calibration alignment by the pattern generator. Accordingly, there exists a need in the art to overcome the deficiencies and limitations described hereinabove. 
       SUMMARY OF THE INVENTION 
       [0010]    In a first aspect of the invention a method comprises determining defect types and defect locations on a mask blank, storing the defect types and the defect locations on the mask blank and generating at least one alignment mark on the mask blank. Additionally, the method comprises selecting a mask pattern for the mask blank based on the defect types and the defect locations and determining a positioning of the mask pattern on the mask blank. Furthermore, the method comprises aligning a mask pattern generator with the mask blank in accordance with the positioning using at least one alignment mark and forming the mask pattern in the mask blank using the mask pattern generator. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The present invention is described in the detailed description which follows, in reference to the noted plurality of drawings by way of non-limiting examples of exemplary embodiments of the present invention. 
           [0012]      FIG. 1  shows two mask substrates containing defects in accordance with the invention; 
           [0013]      FIG. 2  shows two resulting photomasks having different photomask patterns in accordance with the invention; 
           [0014]      FIG. 3  shows two resulting photomasks having different photomask patterns in accordance with the invention; 
           [0015]      FIG. 4  shows two resulting photomasks having a photomask pattern showing a rotation of the photomask pattern in accordance with the invention; 
           [0016]      FIG. 5  shows a resulting photomask having alignment marks according to an aspect of the invention; 
           [0017]      FIG. 6  shows a comparison of a resulting photomask formed without using alignment marks with a resulting photomask formed using alignment marks in accordance the invention; and 
           [0018]      FIG. 7  shows a flow diagram implementing processes in accordance with aspects of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    The present invention relates to a method and system for pre-alignment marking and inspection, and more particularly, to a method and system for pre-alignment marking and inspection to improve mask substrate defect tolerance. 
         [0020]    According to an aspect of the invention, a photomask blank inspection tool may be used to mark alignment features in a non-critical region of the photomask blank to allow a higher calibration alignment by a photomask pattern generator. In embodiments, prior to photomask pattern generation, alignment marks may be scribed or printed on the photomask blank. That is, a photomask blank inspection tool includes a stylus for drawings, e.g., arrows to indicate a defect. These arrows may be used to later locate those defects using, e.g., a scanning electron microscope (SEM). 
         [0021]    According to an aspect of the invention, the stylus of the photomask blank inspection tool may be used to generate the alignment marks. Using these alignment marks and photomask blank inspection data, the photomask pattern may be more accurately positioned on the photomask blank to avoid printing critical features of the photomask pattern in regions impacted by substrate defects. By implementing the invention, extremely accurate overlay results enable the dispositioning techniques to use actual photomask pattern data as opposed to pattern density-based statistics. 
         [0022]    According to an aspect of the invention, a photomask blank inspection tool may inspect photomask blanks upon arrival at a photomask patterning facility. More specifically, photomask blanks that have been coated with absorbing and imaging films may be inspected for defects prior to photomask pattern generation. This inspection may be used to determine locations and severities of defects in the photomask blanks. Additionally, in embodiments, photomask blank defect types and defect locations may be determined by, e.g., a supplier or a manufacturer of photomask blanks, and defect type and location information may be supplied with the shipment of photomask blanks. 
         [0023]    Additionally, according to an aspect of the invention, the photomask blank inspection tool may be used to mark or create alignment features in a non-critical region of the photomask blank to allow a higher calibration alignment by the pattern generator. For example, a LASERTEC inspection tool may be used to generate alignment marks on the photomask blank or substrate. (LASERTEC is a registered trademark of Lasertec Corporation in the United States.) Then, to enable higher resolution coordinate matching, the photomask pattern generator may align to the Lasertec marks formed in the photomask blank. Moreover, based on the improved overlay capability provided by marking the photomask blanks at inspection, photomask pattern formation on the photomask blank may be based on actual computer aided transcription system (CATS) pattern to enable a “tighter fit” of existing defects into non-sensitive regions of the design. 
         [0024]    With the known defect locations from a photomask blank defect inspection tool and the alignment features in the photomask blank created by the photomask blank defect inspection tool, the photomask pattern data can be accurately positioned such that the defects in the photomask blank are in non-critical areas of the photomask pattern or are suitable for easy repair after photomask patterning. For example, the pattern data can be translated, rotated, or magnified slightly to avoid known defects. Moreover, critical areas of the mask pattern can be identified to determine how to change the pattern to more efficiently avoid the effects of mask blank defects. That is, if defects are discovered in the photomask substrate during inspection, this information may be used to determine in real-time what pattern to print on the photomask substrate. 
         [0025]    Moreover, pattern information such as do-inspect regions (DIRs), do-not-inspect regions (DNIRs) and pattern density may be used to estimate the likelihood of an incoming photomask substrate defect occurring in an area that critical pattern is likely to be affected. Based on these results, photomask blanks may be matched with photomask pattern designs that will minimize the occurrence of defects in critical areas that will cause scrap or require repair. That is, for example, some of these printing defects may be located in regions that are not critical. If these blank defects become defects in the photomask, as these defects in the photomask are located in regions that are not critical, the photomask defects may be repairable. With this approach, the effective yield of mask blank fabrication will be increased and the cost of mask production will be reduced. 
         [0026]      FIG. 1  shows two photomask blanks or substrates  100  and  110  having a plurality of defects  105  (represented throughout  FIGS. 1-5  as “*”). These defects  105  have been detected, e.g., by a photomask inspection tool and the positions of the defects  105  have been stored, e.g., in a database. 
         [0027]      FIG. 2  shows the two photomask blanks  100  and  110  with photomask patterns  115  and  120  formed on the photomask blanks  100  and  110 , respectively to form resultant photomasks  160  and  165 , respectively. As shown in  FIG. 2 , the photomask pattern  115  formed on photomask blank  100  is positioned such that some of the defects  105  are contained within the photomask pattern  115 . Thus, using photomask pattern  115  with the photomask blank  100  may render the resulting photomask  160  defective. Furthermore, using the resulting photomask  160  in a photolithography process may cause defects in an integrated circuit. 
         [0028]    In contrast, the photomask pattern  120  formed on photomask blank  110 , is positioned such that the region of the photomask pattern  120  does not contain any of the defects  105  therein. Thus, the defects  105  of the resulting photomask  165  will not produce defects in a formed device, e.g., an integrated circuit, when used during a photolithography process. 
         [0029]      FIG. 3  shows the resulting photomask  160  with the photomask pattern  115  formed on the photomask blank  100 . As shown, the region of the photomask pattern  115  contains many defects  105 . Additionally,  FIG. 3  shows a resulting photomask  170  comprising the same photomask blank  100  (having the same defects  105 ) with a different photomask pattern  140  formed thereon. The photomask pattern  140  includes a “do not inspect region” (DNIR)  135 , which may be, e.g., a non-critical portion of the photomask pattern  140 . As the DNIR  135  is a non-critical portion of the photomask pattern  140 , the defects  105  within this DNIR  135  will not affect the resulting photomask  170 . Thus, as shown in  FIG. 3 , by choosing and positioning a suitable photomask pattern  140 , e.g., having a DNIR  135 , for a particular photomask blank  100 , the number of defects  105  that are present outside of the DNIR  135  may be minimized or eliminated. That is, using photomask pattern  115  on the photomask blank  100  results in five defects being contained in the photomask pattern region. In contrast, using photomask pattern  140  (having DNIR  135 ) on the photomask blank  100  minimizes the number of defects contained in the photomask pattern region that may cause problems during a photolithography process to one. 
         [0030]      FIG. 4  shows how rotating a photomask blank may allow for better positioning of a photomask pattern on the photomask blank, wherein the number of defects contained in the photomask pattern region that may cause problems during a photolithography process is minimized. Thus, as shown in  FIG. 4 , resulting photomask  175  is formed with photomask pattern  140  on photomask substrate  100 ′. As shown in  FIG. 4 , the photomask  175  contains three defects  105  of concern in regions of the photomask pattern  140 . However, by forming the same photomask pattern  140  on a rotated photomask blank  100 , the number of defects  105  of concern (i.e., not within, e.g., a non-critical region of the photomask pattern  140 ) in the resulting photomask  180  can be minimized. 
         [0031]      FIG. 5  shows a resulting photomask  185  formed using alignment marks  205  in accordance with the present invention. More specifically, the photomask blank  200  contains defects  105 . As, described above, these defects  105  may be detected using the photomask blank inspection tool. Additionally, in accordance with an aspect of the invention, during the photomask inspection process using the photomask blank inspection tool, the photomask blank inspection tool is additionally used to scribe alignment marks  205  on the photomask blank. In embodiments, these alignment marks  205  may be formed in a non-critical region of the photomask blank. While the exemplary embodiment shown in  FIG. 5  includes two alignment marks  205 , it should be understood that the invention contemplates that any number of alignment marks  205  may be formed using the photomask blank inspection tool. 
         [0032]    Thus, when the photomask pattern  140  is formed on the photomask blank  200 , a photomask pattern generator may utilize the alignment marks  205  to more precisely position the photomask pattern  140  on the photomask blank  185 . While the photomask blank  200  includes two alignment marks  205  in the exemplary embodiment of  FIG. 5 , it should be understood that the photomask pattern generator may utilize less than the total number of alignment marks  205  present on photomask blank in determining a proper positioning of the photomask pattern  140 . 
         [0033]      FIG. 6  shows an exemplary comparison of a photomask pattern  310  formed on a photomask substrate  315  without any alignment marks with the photomask pattern  310  formed on a photomask blank  315 ′ which has alignment marks  320  in accordance with the present invention. As shown in  FIG. 6 , with the resulting photomask  300  the photomask pattern  310  has been formed on the photomask blank  315  in regions containing a defect  305 . 
         [0034]    Additionally, as shown in  FIG. 6 , photomask blank  315 ′ includes marking  320  formed on the photomask blank  315 ′ using the photomask blank inspection tool. In accordance with the present invention, the photomask pattern generator may be aligned with the alignment marks  320  when the photomask pattern  310  is formed on the photomask blank  315 ′ to more precisely position the photomask pattern  310 . Thus, as shown in  FIG. 6 , in contrast to resulting photomask  300 , with the resulting photomask  300 ′, the photomask pattern generator has been aligned with the alignment marks  320  to more precisely position the photomask pattern  310  on the photomask blank  315 ′. Thus, in the exemplary embodiment of  FIG. 6 , the photomask pattern  310  is formed on the photomask blank  315 ′ in regions of the photomask blank  315 ′ that are free of defects. 
       Flow Diagram 
       [0035]      FIG. 7  shows an exemplary flow  700  in accordance with the present invention.  FIG. 7  equally represents a high-level block diagram of the invention. At step  705 , a photomask blank is inspected to detect defects using a photomask blank inspection tool. Photomask blank defect inspection information is compiled, including the detected defect locations and defect types. At step  710 , an inventory of inspected photomask blanks is created and the photomask blank inspection information is fed forward for analysis. At step  715 , a photomask pattern is selected, e.g., as requested from a customer. At step  717 , a photomask blank is selected for analysis. At step  720 , a likelihood of a photomask blank defect impacting the photomask pattern is determined, e.g., an area within a certain radius. At step  725 , defect size and type are accounted for relative to the photomask pattern. At step  730 , the defect locations are compared to non-critical regions, e.g., DNIRs, of the photomask pattern. At step  735 , areas on a photomask blank where defects would fall within non-critical regions of the photomask pattern are determined. At step  740 , layer sensitivities are accounted for, including the capacity of the layer to absorb damage (e.g., turn-around time (TAT) commitments, etc.). At step  745 , the repair difficulty of a resulting photomask is determined. At step  750 , orientations (e.g., 3, 6, 9 or 12 o&#39;clock) of the photomask blank for patterning are determined. At step  755 , a determination as to the suitability of the photomask blank for the photomask pattern is determined based on the determinations made at one or more of steps  720 - 750 . 
         [0036]    If, at step  755 , the photomask blank is determined to be unsuitable for the photomask pattern due to the defects, the process continues at step  715 , where another photomask blank is selected for analysis for the photomask pattern. If, at step  755 , the photomask blank is determined to be suitable for the photomask pattern, at step  760 , the selected photomask blank is used. At step  765 , alignment marks are generated on the photomask blank using the photomask inspection tool to enable higher resolution coordinate matching. At step  770 , the alignment marks are used to align the photomask pattern generator. At step  775 , the selected photomask pattern is formed on the photomask blank using the photomask generator. 
         [0037]    It should be understood, that while the steps have been described as occurring in a particular order, the invention contemplates that the steps may be performed in other orders. For example, step  770  may occur prior to step  710 . Additionally, for example, step  750  may occur prior to step  740 . Also, steps  725 ,  730  and  735  may be performed in a different order than shown in  FIG. 7 . Furthermore, the invention contemplates that, in embodiments, steps may be implied or omitted while still remaining true to this invention. 
         [0038]    While the invention has been described in terms of embodiments, those of skill in the art will recognize that the invention can be practiced with modifications and in the spirit and scope of the appended claims.