Abstract:
A customer illumination aperture (CIA) structure for lithographic exposure is disclosed, including a central part and at least one off-axis part around the central part. The off-axis part of the CIA is disposed in a symmetric manner with respect to the central part.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to the lithographic technology. More particularly, the present invention relates to a customer illumination aperture (CIA) structure for lithographic exposure, which can improve the resolution in simultaneous formation of dense, semi-dense and isolated patterns.  
         [0003]     2. Description of the Related Art  
         [0004]     In advanced semiconductor processes, especially those of 90 nm generation or below, resolution-enhanced technologies (RET) are required to achieve fine pitch resolution. Many methods have been proposed to overcome the issue of lower k1 value, such as, the methods using alternating phase-shift masks (Alt-PSM), chromeless masks, vertex masks or half-tone masks in exposure, and multiple exposure methods.  
         [0005]     Other RET methods include judicious applications of mask biasing and inclusion of additional assisting features. However, these methods suffer from high cost and low throughput on mask manufacturing since circuit designs are often too complex to be handled by optical proximity correction (OPC) software.  
         [0006]     Another RET method is to use optimal customer illumination apertures (CIAs), as described in U.S. Pat. No. 6,839,125. For example, the illumination aperture of  FIG. 1  having one ring-shaped off-axis part  110  is often applied between the exposure light source and the photomask. However, such an illumination aperture is not so good in definition of isolated and semi-dense patterns.  
       SUMMARY OF THE INVENTION  
       [0007]     In view of the foregoing, this invention provides a customer illumination aperture (CIA) structure for lithographic exposure, which can improve the resolution in simultaneous formation of dense, semi-dense and isolated patterns.  
         [0008]     The CIA structure of this invention includes a central part and at least one off-axis part around the central part. The at least one off-axis part is disposed in a symmetric manner with respect to the central part.  
         [0009]     In the above CIA structure of this invention, the central part may have a ring shape. The off-axis part may be a single region having a ring shape, or includes a number “n” (n≧2) of regions arranged in n-fold symmetry around the central part. In addition, there may be multiple off-axis parts arranged in two or more circles around the central part. Combinations of two or all of the three additional features are also allowed.  
         [0010]     Since the central part is good for definition of isolated and semi-dense patterns and the off-axis part good for that of dense patterns, the overall quality of pattern transfer can be improved. By comparing the simulated aerial images and the real resist profiles obtained in experiments, it is confirmed that the through-pitch CD uniformity, mask error enhancement factor (MEEF), line-end shortening problem, pattern linearity and depth of focus (DOF) can be improved by using the illumination aperture of this invention with reduced OPC loading and cost.  
         [0011]     It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  illustrates a CIA structure in the prior art.  
         [0013]      FIGS. 2-4  illustrate three examples of CIA structure of this invention, wherein each off-axis part is a single region having a ring shape.  
         [0014]      FIGS. 5A-5D  illustrate four more examples of CIA structure of this invention, wherein the off-axis part include n regions (n≧2) arranged in n-fold symmetry.  
         [0015]     FIGS.  6  illustrates another exemplary CIA structure of this invention, wherein the off-axis part includes 3 regions arranged in 3-fold symmetry that are connected with the central part. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]      FIGS. 2-4  illustrate three examples of CIA structure of this invention, wherein each off-axis part is a single region having a ring shape.  
         [0017]     Referring to  FIG. 2 , the CIA structure of this example includes a solid central part  200  and an off-axis part  210 , which is a single region having a ring shape around the central part  200 . Referring to  FIG. 3 , the central part  200  in the CIA structure may also have a ring shape.  
         [0018]     Moreover, there may be more than one off-axis parts arranged in two or more circles around the central part  200 , wherein any off-axis part can be a single region having a ring shape or includes n regions (n≧2) arranged in n-fold symmetry. As shown in  FIG. 4 , there are a first off-axis part  210  around the central part  200  and a second off-axis part  220  around the first one  210 , wherein each of the first and second off-axis parts  210  and  220  can be a single region having a ring shape.  
         [0019]      FIGS. 5A-5D  illustrate four more examples of CIA structure of this invention, wherein the off-axis part include n regions (n≧2) arranged in n-fold symmetry. The n regions are preferably shaped such that they can be covered by an imaginary ring region around the central part  200 , and n is preferably an integer between 2 and 6. For example, the two, three, four or six regions of the off-axis part  210  in  FIG. 5A, 5B ,  5 C or  5 D can be covered by an imaginary ring region corresponding to the off-axis part  210  of single-ring shape in  FIG. 2 . In real applications, the n regions (n≧2) of the off-axis part  210  can be made by, for example, forming a ring-shaped aperture and then disposing a corresponding number of screen plates in n-fold symmetry to divide the ring-shaped aperture into n regions, as described in U.S. Pat. No. 6,839,125.  
         [0020]     Referring to  FIG. 5A , the off-axis part  210  includes two regions that are arranged in 2-fold symmetry. The two regions are preferably arranged in x- or y-direction. When the two regions are arranged in x-direction, the x-directional resolution obtained is better than that obtained with the CIA structure of  FIG. 2 , but the y-directional resolution is worse relatively. Moreover, when the x-direction effect is too strong, the hole patterns in the photoresist layer will be distorted to lower the symmetricity thereof.  
         [0021]     Referring to  FIG. 5B , the off-axis part  210  includes three regions that are arranged in 3-fold symmetry. When such a CIA structure is used, both the centroid and the symmetricity of the photoresist patterns can be maintained well.  
         [0022]     Referring to  FIG. 5C , the off-axis part  210  includes four regions that are arranged in 4-fold symmetry. In the illustrated example, the four regions are arranged in ±y-directions. When such a CIA structure is used, the x-directional resolution obtained is better than that obtained with the CIA structure of  FIG. 2 , but is worse than that obtained with the CIA structure of  FIG. 5A . However, the hole patterns in the photoresist layer will not be distorted when the CIA structure of  FIG. 5C  is adopted, because there is little x-direction effect caused by this CIA structure.  
         [0023]     Referring to  FIG. 5D , the off-axis part  210  includes six regions that are arranged in 6-fold symmetry. The width of one region in the circumferential direction of the circle is preferably larger than the distance between two regions.  
         [0024]     Furthermore, when the off-axis part include n regions in n-fold symmetry, the n regions of the off-axis part may be connected with the central part of the CIA structure, as shown in  FIG. 6 , where the off-axis part  210  include three regions in 3-fold symmetry that are connected with the central part  200 .  
         [0025]     In addition, any combination of two or al! of the three features about the shape of the central part  200 , the number of off-axis parts and the geometry (single or n regions) of an off-axis part is also possible, depending on the shapes and arrangements of the patterns to be transferred to the photoresist layer.  
         [0026]     It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention covers modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.