Abstract:
A photomask that includes an assistant pattern is provided. The photomask comprises a target pattern transcribed over a wafer by an exposing process, and an assistant pattern formed symmetrically with a main pattern of the target pattern based on the outer pattern of the target pattern, thereby minimizing the loss of the outer pattern and maximizing the process margin in the defocus environment.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The priority of Korean patent application No. 10-2009-0012855 filed Feb. 17, 2009, the disclosure of which is hereby incorporated in its entirety by reference, is claimed. 
       BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to a photomask used in an exposing process for manufacturing a semiconductor device, and more specifically, to a photomask that includes an assistant pattern formed in an open region around the main pattern. 
         [0003]    In photolithography for manufacturing a semiconductor device, a photomask is used to form a pattern on a semiconductor substrate. The photomask has a mask pattern for forming various components of the semiconductor device. As the semiconductor device becomes more integrated, the feature size of the mask pattern becomes smaller. 
         [0004]    When the feature size of the mask pattern reaches a resolution limit of an exposer, it becomes difficult to transfer a desired pattern on a substrate due to the optical proximity effect. 
         [0005]    That is, when the mask pattern is transferred on a substrate by illumination of exposure sources (e.g., KrF excimer laser or ArF excimer laser), the pattern formed on the photomask is not transferred to the substrate uniformly. The pattern is distorted depending on the its location of the pattern. When the light transmitted by a light source passes through the photomask, various optical phenomena occur depending on the location and the shape of the mask pattern so that the light intensity varies depending on the location on the photomask. 
         [0006]    In order to solve the above problem, an optical proximity correction (OPC) technology has been used. 
         [0007]    The OPC is to calculate statistically or experimentally a relation between an intended pattern and an actual pattern formed in photoresist. The size and shape of the mask pattern can then be adjusted depending on the calculation. 
         [0008]    However, when a conventional OPC is used, it is difficult to improve the margin of the depth of focus and the uniformity of a critical dimension (CD) in all of the patterns of the chip. A pattern formed at an outer edge of a cell array is particularly vulnerable to defocus. 
         [0009]      FIG. 1  is a diagram illustrating a SEM photograph of contact holes formed in a cell array region when optical proximity correction is performed without an assistant pattern. 
         [0010]    Referring to  FIG. 1 , a contact hole pattern disposed in the outer edge is distorted due to the defocus phenomenon.  FIG. 1  shows how the defocus phenomenon changes in the outer edge pattern as the depth of focus changes from −0.2 μm to −0.04 μm. 
         [0011]      FIG. 2  is a diagram illustrating an aerial image of the contact hole pattern of  FIG. 1  with the best focus. 
         [0012]    As shown in an aerial image  12  disposed between contact holes  11  of  FIG. 2 , when defocus occurs, likelihood of generating bridges between the adjacent contact holes  11  increases significantly. 
         [0013]    In order to prevent the generation of bridges, a method of enlarging a hole diameter of the outer edge pattern has been used. However, when the diameter of the contact hole is enlarged, a contact region encroaches into a gate region and may cause a short between the two regions. As a result, the method of enlarging a hole diameter of the outer edge pattern has a limit. 
         [0014]    Alternatively to the solution above, a method of inserting an assistant pattern into a photomask may be used. However, it is difficult to obtain a desired outer pattern and a desired process margin by this method. 
         [0015]    As a result, a new assistant pattern is required to obtain a desired process margin for forming the outer pattern, which is most vulnerable to the defocus phenomenon. 
       BRIEF SUMMARY OF THE INVENTION 
       [0016]    Various embodiments of the invention are directed to providing a photomask that comprises an assistant pattern formed symmetrically with a main pattern with the same pitch. 
         [0017]    According to an embodiment of the present invention, a photomask comprises: a target pattern transferred over a substrate by photolithography; and an assistant pattern formed symmetrically with a main pattern of the target pattern based on the outer pattern of the target pattern. 
         [0018]    The assistant pattern is formed symmetrically with the main pattern, thereby improving the process margin of the outer pattern as well as the main pattern and reducing the loss of the outer pattern in a defocus environment. 
         [0019]    Preferably, the assistant pattern has the same pitch as that of the main pattern. The assistant pattern is formed to have the same pitch as that of the main pattern and to be symmetrical with the main pattern, thereby reducing the loss of the outer pattern in the defocus environment. 
         [0020]    Preferably, the assistant pattern includes first lines and second lines that form a net shape. The first lines have a slope that is substantially the same as a slope of a first set of the main pattern and the second lines have a slope that is substantially the same as a slope of a second set of the main pattern. 
         [0021]    Preferably, the assistant pattern is formed to have the same distance between the adjacent crossed points of the first lines and the second lines as the pitch of the main pattern. The assistant pattern has the same X direction pitch as Y direction pitch between the adjacent points. 
         [0022]    Preferably, the assistant pattern includes a chrome (Cr) film, a molybdenum (Mo) film and a stacked structure thereof. 
         [0023]    Preferably, the assistant pattern is a brick wall shaped pattern in an open region adjacent to the outer pattern. 
         [0024]    Preferably, the main pattern and the outer pattern are contact hole patterns formed repeatedly with a given interval in a cell array region. The target pattern is an optical-proximity-corrected pattern. The outer pattern has a critical dimension larger than that of the main pattern 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]      FIG. 1  is a diagram illustrating a SEM photograph of contact holes formed in a cell array region when optical proximity correction is performed without an assistant pattern. 
           [0026]      FIG. 2  is a diagram illustrating an aerial image of the contact hole pattern of  FIG. 1  in the best focus. 
           [0027]      FIG. 3  is a diagram illustrating a layout of a photomask according to an embodiment of the present invention. 
           [0028]      FIG. 4   a  is a diagram illustrating an aerial image when an exposing process is performed with a photomask that comprises an assistant pattern according to an embodiment of the present invention. 
           [0029]      FIG. 4   b  is a diagram illustrating an aerial image when an exposing process is performed with a photomask that does not comprise an assistant pattern according to an embodiment of the present invention. 
           [0030]      FIG. 5   a  is a graph illustrating the intensity of light irradiated along A-A′ in the aerial image of  FIG. 4   a.    
           [0031]      FIG. 5   b  is a graph illustrating the intensity of light irradiated along B-B′ in the aerial image of  FIG. 4   b.    
           [0032]      FIG. 6  is a diagram illustrating the aerial image around the outer pattern in  FIG. 4   a.    
           [0033]      FIG. 7  is a diagram illustrating assistant patterns according to another embodiment of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0034]      FIG. 3  is a diagram illustrating a layout of a photomask according to an embodiment of the present invention. 
         [0035]    The photomask of  FIG. 3  comprises a target pattern  100  and an assistant pattern  200  which are formed over a transparent substrate (not shown). 
         [0036]    The target pattern  100  a pattern to be transferred to a photoresist film by photolithography. In the present embodiment, the target pattern  100  is a pattern for contact holes to be formed in a cell array region. The target pattern  10  of  FIG. 3  shows a pattern shape obtained by performing an optical proximity correction on the contact hole pattern formed with a given pitch in the cell array region. A pattern  120  formed on the outside edge of the target pattern  100  (hereinafter, referred to as “outer pattern”) is a dummy pattern for improving a process margin of a main pattern  110 . The outer pattern  120  is formed to have a hole diameter larger than that of the main pattern  110 . 
         [0037]    The assistant pattern  200  is formed on the photomask, but does not get transferred to the photoresist. The assistant pattern  200  is formed in an unused region (open region) next to the outer pattern  120 , thereby maximizing the optical effect of the target pattern  100  to improve the process margin of the target pattern  100 . The assistant pattern  200  is formed to have a critical dimension smaller than a resolution of an illuminator used in an exposing process, so that so that the assistant pattern  200  is not transferred to the photoresist during the photolithography process. The assistant pattern  200  is formed “symmetrically” with (or to resemble) the main pattern  110  with the the outer pattern  120  as a reference line. This is done in order to improve the process margin of the target pattern  100 , particularly, the outer pattern  120 . As used herein, the term “symmetrical” or “symmetrically” refers the patterns that have the same X-direction and Y-direction pitches. In the present embodiment, the assistant pattern  200  and the main pattern  110  have the same pitch for their respective iterations of shapes. The assistant pattern  200  is preferably formed in a region of larger than 1 μm from the outer pattern  120 . 
         [0038]    The assistant pattern  200  includes line patterns  210  arranged in parallel to the contact hole patterns of the main pattern  110 . The line patterns  210  cross each other diagonally to create a net shape. In the assistant pattern  200  of  FIG. 3 , distances between points where the lines intersect [i.e., a X direction pitch (X′) and a Y direction pitch (Y′)] are formed to be the same as a X direction pitch (X) and a Y direction pitch (Y) of the main pattern  110 . Although  FIG. 3  shows the pitch X′ is formed to be the same as the pitch Y′, they may be different from each other depending on the shape of the main pattern  110 . 
         [0039]    The target pattern  100  and the assistant pattern  200  may be formed, respectively, of a chrome (Cr) film, a molybdenum (Mo) film and a stacked structure thereof. 
         [0040]      FIG. 4   a  is a diagram illustrating an aerial image when an exposing process is performed with a photomask that includes an assistant pattern according to an embodiment of the present invention.  FIG. 4   b  is a diagram illustrating an aerial image when an photolithography is performed with a photomask that does not include an assistant pattern according to an embodiment of the present invention. 
         [0041]    The comparison of the aerial images of  FIGS. 4   a  and  4   b  is as follows. 
         [0042]    Referring to  FIG. 4   b,  an aerial image of the main pattern  13  and an aerial image of the outer pattern  14  are formed. Referring to  FIG. 4   a,  an aerial image  410  symmetrical with the aerial image  310  of the main pattern is formed in an open region outside of the outer pattern. That is, although the assistant pattern  210  is not transferred onto the semiconductor substrate, the aerial image  410  symmetrical with the aerial image  310  is formed in the open region based on the aerial image of the outer pattern  320 . 
         [0043]      FIG. 5   a  is a graph illustrating the intensity of light irradiated along A-A′ in  FIG. 4   a,  and  FIG. 5   b  is a graph illustrating the intensity of light irradiated along B-B′ in  FIG. 4   b.    
         [0044]      FIG. 5   b  shows main peaks  15  and an outer peak  16  corresponding respectively to a point where a main pattern  13  is supposed to form and a point where an outer pattern  14  is supposed to form, but no aerial image peak exists in the open region. The outer peak  16  has about the same height as the main peak  15 , so that their peak values are similar to each other. It is difficult to improve the process margin of the outer pattern  120  in the exposing process. 
         [0045]    On the other hand,  FIG. 5   a  shows an assistant peaks  530  that have a similar shape to that of the main peak  510  although the peak value of the assistant peak  530  is small. That is, the assistant peaks  530  corresponding to the assistant pattern  200  are symmetrical with the main peaks  510  of the main pattern  110  with respect to the outer peak  520  of the outer pattern  120  as a centerline (or a reference). The value (i.e., 0.42) of the outer peak  520  corresponding to the outer pattern  120  is significantly larger than the value (i.e., 0.35) of the main peak  510 . As shown in  FIG. 5   a,  the formation of the assistant pattern  200  increases the value of the outer peak  520  of the outer pattern  120 . 
         [0046]    In other words, the assistant pattern  200  that has the same pitch as that of the main pattern  110  is formed to be symmetrical with the main pattern  110 , thereby increasing the difference between the light intensity for the main pattern  110  and the assistant pattern  200 . As a result, the value of the outer peak  520  of the outer pattern  120  disposed between the main pattern  110  and the assistant pattern  200  is increased. 
         [0047]    In this way, the increase of the outer peak  520  also increases the process margin of the outer pattern  120 . The outer pattern  120  is thereby made resistant to the defocus phenomenon and is formed to have a desired shape. 
         [0048]      FIG. 6  is a diagram illustrating the aerial image around the outer pattern  320  in  FIG. 4   a.    
         [0049]    As shown in the aerial image  330  formed between the contact holes  320  of the outer pattern of  FIG. 6 , the possibility of generation of bridges between the adjacent contact holes  320  is lowered in comparison with  FIG. 2 . 
         [0050]      FIG. 7  is a diagram illustrating assistant patterns according to another embodiment of the present invention. 
         [0051]    The assistant pattern of  FIG. 3  includes diagonal line patterns  210  which are crossed to have a net shape. However, the assistant pattern may be formed to include line patterns arranged in a horizontal and vertical direction to have a checker board shape (the pitch depends on the shape of the main pattern  110 ) as shown in  7   a,  or a brick wall shape as shown in  FIG. 7   b.  The brick wall shape means a shape of bricks which are laid like a wall. 
         [0052]    The above embodiments of the present invention are illustrative and not limitative. Various alternatives and equivalents are possible. The invention is not limited by the type of deposition, etching polishing, and patterning steps describe herein. Nor is the invention limited to any specific type of semiconductor device. For example, the present invention may be implemented in a dynamic random access memory (DRAM) device or non volatile memory device. Other additions, subtractions, or modifications are obvious in view of the present disclosure and are intended to fall within the scope of the appended claims.