Abstract:
The invention relates to a photomask for use in a thermal flow process in which: a photomask is prepared in which a plurality of exposure openings are formed; a resist is applied to the surface of a layer of a semiconductor integrated circuit that is to undergo processing; this resist is patterned by an exposure process through the photomask to form a plurality of openings in the resist that correspond to each of the exposure openings; and the patterned resist is then heated to cause each of the openings to shrink; wherein at least a portion of exposure openings among the plurality of exposure openings are formed in shapes that compensate for anisotropic deformation that occurs in the openings when the patterned resist is heated to cause each of the openings to shrink. Since the openings that are formed in the resist are provided in advance with shapes that compensate for the deformation that occurs when the openings shrink, these openings attain the proper shape after undergoing shrinking and deformation.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a photomask used in a thermal flow process, a method of forming patterns used in a thermal flow process, and a semiconductor integrated circuit in which a portion having fine planar shapes is treated by a prescribed process through openings in a resist.  
           [0003]    2. Description of the Related Art  
           [0004]    In recent years, fine-patterned semiconductor integrated circuits constructed by using thin-film techniques are being used for a variety of purposes, and these constructions are increasing in fineness with each year. As an example, photolithography is one technique for achieving fine patterning of the layers of a semiconductor integrated circuit.  
           [0005]    In a case of forming through-holes in the insulating film of a semiconductor integrated circuit, a resist is applied to the surface of the insulating film that is to undergo processing, and the resist is then exposed using a photomask in which a plurality of exposure openings are formed. The resist is then developed to form openings at the exposed portions, and this resist is used as a mask to etch the insulating film through the openings.  
           [0006]    This type of photolithography is used not only for the formation of through-holes described above but for various other purposes such as introducing impurities into a semiconductor substrate and patterning wiring lines. In this type of photolithography, a photomask is formed in which the pattern that is to be exposed is enlarged in all directions, following which the exposure process is carried out with this photomask using reducing optics to expose a pattern of the desired dimensions on the resist.  
           [0007]    In this technique, a pattern that is finer than a prescribed dimension cannot be exposed due to the limits of optical resolution. However, there is now demand to reduce resist openings below the exposure limit dimensions, and the thermal flow process has been developed as a means of realizing such a reduction.  
           [0008]    Referring now to FIG. 1A-FIG. 2C, one example of the thermal flow process of the prior art is next described.  
           [0009]    As shown in FIG. 1A, a DRAM (Dynamic Random Access Memory) that is in the process of fabrication is first prepared as semiconductor integrated circuit  100 , which is the object of processing. In semiconductor integrated circuit  100  that is here taken as an example, gate oxide film  102  is formed on the surface of semiconductor substrate  101 , and gate electrodes  103  and  104  of the transistor elements that will serve as memory cells are formed in a prescribed pattern on the surface of this gate oxide film  102 . Gate oxide film  102  is partitioned by element isolation regions  105  according to the positions of memory cells, and the space around gate electrodes  103  and  104  is filled with interlayer dielectric film  106 , which is a prescribed layer.  
           [0010]    In semiconductor integrated circuit  100  which is taken as an example here, contact hole  107  of a bit contact is formed from the surface of interlayer dielectric film  106  to the surface of gate oxide film  102  at a position between the pair of gate electrodes  103  and  104 , as shown in FIG. 2C. Photomask  111 , in which is formed exposure opening  110  that corresponds to this contact hole  107 , is therefore prepared as shown in FIG. 1C.  
           [0011]    The structure of this photomask  111  is such that shield film  113  is formed on the underside of transparent base member  112  and exposure opening  110  is formed by partially removing this shield film  113 . This exposure opening  110  is formed at position that corresponds to contact hole  107 , and its dimensions in all directions are greater than the dimensions of contact hole  107 .  
           [0012]    Resist  115  is then applied to the surface of interlayer dielectric film  106 , which is a prescribed layer of semiconductor integrated circuit  100 , to form a prescribed film thickness as shown in FIG. 1B, and the above-described photomask  111  is arranged parallel to and confronting the surface of resist  115  at a prescribed distance from the surface of resist  115 .  
           [0013]    In this configuration, resist  115  is exposed to light by exposure device (not shown in the figure) through exposure opening  110  of photomask  111 , and as shown in FIG. 2A, this resist  115  is then developed to form opening  116  that corresponds to exposure opening  110 . In the photolithographic technique of the prior art, a contact hole is formed in interlayer dielectric film  106  of semiconductor integrated circuit  100  through this opening  116  in resist  115 .  
           [0014]    However, since it is impossible to form contact hole  107  of a diameter that is still smaller than the dimension limited by exposure resolution, resist  115  that has been patterned as described hereinabove is heated and softened in a thermal flow process to shrink opening  116  as shown in FIG. 2B.  
           [0015]    Since opening  116  of resist  115  thus attains a diameter that is smaller than the exposure limit dimension, an extremely small diameter contact hole  107  can be formed from the surface of interlayer dielectric film  106  to the surface of gate oxide film  102  by etching interlayer dielectric film  106  of semiconductor integrated circuit  100  through opening  116  in resist  115 .  
           [0016]    Exposure opening  110  of photomask  111 , which is used in the exposure process in the above-described thermal flow process, is therefore formed at dimensions that approach the limit dimensions of the exposure process and in a shape that is an enlargement in all directions of opening  116  that has been shrunk by heating resist  115 .  
           [0017]    When the dimensions of exposure opening  110  approach the exposure limit dimensions, the shape of an exposure beam that passes through exposure opening  110  is deformed by such factors as diffraction. The shape of the exposure of opening  116  in resist  115  is therefore roughly oval in shape even though exposure opening  110  is square, and the shape of opening  116  following the thermal flow process becomes approximately circular.  
           [0018]    Since no problem is raised if the plan shape of contact hole  107  that is formed at the exposure limit dimensions as described above is substantially circular, exposure opening  110  of photomask  111  is typically formed as a square in order to simplify design and fabrication. As a result, in a case in which the exposure dimension of opening  116  in resist  115  is set to a circle of diameter “a”, exposure opening  110  of photomask  111  is formed as a square having sides of length “a”.  
           [0019]    In the interest of simplifying the explanation here, a case is described in which the process of exposing resist  115  using photomask  111  is carried out in equal proportions, but in a case in which the exposure process is performed in the above-described exposure limit dimensions, the pattern of openings of photomask  111  is typically exposed on resist  115  in a form that is reduced by reduction optics.  
           [0020]    After forming opening  116  in resist  115  by an exposure process using photomask  111  in the above-described thermal flow process, this resist  115  is heated to shrink opening  116 , whereby a process can be performed on interlayer dielectric film  106  at dimensions that are smaller than the exposure limit dimension.  
           [0021]    Nevertheless, when resist  115  is heated and softened to shrink opening  116  that was formed by the exposure process as described hereinabove, opening  116  deforms as it shrinks due to the surface tension of this resist  115 . It has been confirmed by the inventors of this invention that this deformation occurs in accordance with the positional relationships between the plurality of openings  116 . Specifically, when shrinking a plurality of openings  116  by heating resist  115 , the degree of shrinkage at each of openings  116  that are close to each other is smaller in the direction between openings  116  while the degree of shrinking is greater in the direction orthogonal to this direction.  
           [0022]    In some types of high-integration DRAM referred to as “¼pitch DRAM,” for example, a plurality of contact holes  107  are arranged linearly in a direction that is inclined 45° from the directions of arrangement of the bit lines and word lines. Photomask  111  for forming such a plurality of contact holes  107  has a shape in which a plurality of square exposure openings  110  are arranged in a line in a 45° direction, as shown in FIG. 3A.  
           [0023]    When resist  115  is exposed using this type of photomask  111 , a plurality of round openings  116  is thus arranged in a line in a 45° direction, as shown in FIG. 3B. When this resist  115  is heated and openings  116  are caused to shrink, however, the degree of shrinkage in the direction of arrangement is small, while the degree of shrinkage in the direction orthogonal to this direction is great, and, as shown in FIG. 3C, each of openings  116  therefore forms an oval that is elongated in the direction of arrangement of openings  116 .  
           [0024]    In a thermal flow process of the prior art, the formation of openings  116  of a desired shape in desired positions is problematic due to deformation according to the positional relationship between the plurality of openings  116 , as described in the foregoing explanation, and the proper realization of prescribed fine processing on semiconductor integrated circuit  100  is therefore also problematic.  
         SUMMARY OF THE INVENTION  
         [0025]    It is an object of the present invention to provide a photomask that can properly realize prescribed fine processing on a semiconductor integrated circuit in a thermal flow process; a pattern forming method that can properly realize prescribed fine processing on a semiconductor integrated circuit in a thermal flow process; and a semiconductor integrated circuit in which prescribed fine processing is properly realized.  
           [0026]    According to one aspect of the present invention, a photomask is used in a thermal flow process in which: a photomask is prepared in which a plurality of exposure openings are formed; a resist is applied to the surface of the layer of a semiconductor integrated circuit that is to undergo processing; the resist is patterned by an exposure process through the photomask to form a plurality of openings in the resist that correspond to each of the exposure openings; and the resist in which the patterning has been carried out is heated to cause each of the openings to shrink; wherein at least a portion of the exposure openings among the plurality of exposure openings are formed in a shape that compensates for the anisotropic deformation that occurs in the openings when each of the openings is caused to shrink by heating the patterned resist. In the thermal flow process that uses the photomask of the present invention, when the resist that is applied to the surface of a layer of a semiconductor integrated circuit that is to undergo processing is patterned by an exposure process by means of the photomask and a plurality of openings are formed in the resist that correspond to the plurality of exposure openings that are formed in the photomask, these openings are formed in a shape that compensates for the anisotropic deformation that occurs when the resist is heated to cause each of the openings to shrink. When the resist that has been thus patterned is heated and the openings are caused to shrink, these openings are anisotropically deformed as they shrink. However, since each of the openings has been formed in advance in a shape that compensates for this anisotropic deformation, the openings attain the proper shape after shrinkage and deformation.  
           [0027]    At least a portion of exposure openings among the plurality of exposure openings may be formed in a shape that is elongated in a direction that is approximately orthogonal to the direction toward other exposure openings that are close. At least a portion of exposure openings among the plurality of exposure openings may also be enlarged in a direction that is approximately orthogonal to the direction toward other exposure openings that are close. The degree of enlargement of said exposure openings becomes smaller as the distance among said other exposure openings that are close becomes larger. When the plurality of openings that have been formed in this way are caused to shrink by heating the resist, the plurality of openings that neighbor each other attain the proper shape upon shrinking because the degree of shrinkage is smaller in the direction toward other openings while the degree of shrinkage is greater in the direction orthogonal to this direction due to such factors as the surface tension of the resist.  
           [0028]    At least a portion of the exposure openings among the plurality of exposure openings are arranged in lines that are close together, and each of the exposure openings that are arranged in these lines may be enlarged in the direction that is approximately orthogonal to the direction of this arrangement.  
           [0029]    The exposure openings may be formed in a rectangular shape in which the direction of enlargement is the direction in which the long sides extend.  
           [0030]    Each of the exposure openings may be enlarged in substantially all directions, and at least a portion of the exposure openings among the plurality of exposure openings may be formed such that the degree of enlargement is smaller in the direction toward other exposure openings than other directions that are close.  
           [0031]    At least a portion of the exposure openings among the plurality of exposure openings may be formed as rectangles in which the short sides extend in the direction toward other exposure openings that are close and the long sides extend in a direction that is approximately orthogonal to this direction.  
           [0032]    The term “enlargement of the openings in the resist” in this invention assumes the dimensions before the openings are caused to shrink by the thermal flow process with respect to the final desired dimension of the openings that have been caused to shrink by the thermal flow process, and thus indicates that the dimensions of exposure of the resist are made greater.  
           [0033]    For example, if the diameter of round openings that are caused to shrink by the thermal flow process is “a” and the openings are caused to shrink to “1/b” by the thermal flow process, the openings before being caused to shrink by the thermal flow process are circles having a diameter of “a×b,” but in the present invention, the exposure dimension of the resist openings is made “a× b” or greater in the direction of enlargement.  
           [0034]    In addition, the term “enlargement of exposure openings in the photomask” in the present invention means that, when forming openings of a desired dimension in the resist, the exposure openings are made larger than dimensions that are designed based merely on these openings. For example, in a case in which the diameter of circular openings that are caused to shrink by the thermal flow process as described hereinabove is “a” and the openings are caused to shrink to “1/b” by the thermal flow process, the openings before being caused to shrink by the thermal flow process are circles of diameter “a× b.” If the exposure optics are equal power, square exposure openings measuring “a×b” on each side would be formed in the photomask, but in the present invention, the exposure openings that are formed in the photomask are rectangles in which the short sides are “a×b” in length and the long sides are longer than “a×b.”  
           [0035]    Furthermore, “approximately all directions” in the present invention means substantially all directions involved in the formation of the exposure openings and includes 360° of the two-dimensional directions that are parallel to the surface of the photomask, the four directions to the left and right and forward and rear that are parallel to the surface of the photomask, and the two directions that are parallel to the four sides of the exposure openings that are formed in a rectangular shape.  
           [0036]    The pattern forming method according to another aspect of the present invention is a pattern forming method used in a thermal flow process in which: a resist is applied to a surface of the layers of a semiconductor integrated circuit that is to undergo processing; the resist is patterned to form a plurality of openings in the resist; and the resist that has been patterned is heated to cause each of the openings to shrink; wherein at least a portion of exposure openings among said plurality of exposure openings are formed in shapes so that said openings are caused to become corresponding desired shapes due to anisotropic deformation that occurs in said openings when said resist that has been patterned is heated to cause said openings to shrink.  
           [0037]    The pattern forming method according to another aspect of the present invention is a pattern forming method used in a thermal flow process in which: a photomask is prepared in which a plurality of exposure openings are formed; a resist is applied to a surface of the layers of a semiconductor integrated circuit that is to undergo processing; this resist is patterned by an exposure process through the photomask to form a plurality of openings in the resist that correspond to the exposure openings; and the patterned resist is heated to cause each of the openings to shrink; wherein the photomask of the present invention is used during the exposure process.  
           [0038]    According to another aspect of the present invention, a prescribed portion of a semiconductor integrated circuit having fine planar shapes is treated by a prescribed process through openings in a resist that have been formed by the method of forming patterns of the above-described invention.  
           [0039]    The above and other objects, features, and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate examples of the present invention.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0040]    FIGS.  1 A- 1 C and FIGS.  2 A- 2 C are schematic vertical section frontal views showing a semiconductor integrated circuit that is to undergo processing for explaining an example of a thermal flow process of the prior art;  
         [0041]    [0041]FIG. 3A is a plan view showing an example of a photomask of the prior art;  
         [0042]    [0042]FIG. 3B is a plan view showing a resist in which openings have been formed by exposure using the photomask shown in FIG. 3A;  
         [0043]    [0043]FIG. 3C is a plan view showing the state of openings that have been caused to shrink by heating the resist shown in FIG. 3B;  
         [0044]    [0044]FIG. 4A is a plan view showing one embodiment of the photomask according to the present invention;  
         [0045]    [0045]FIG. 4B is a plan view showing a resist in which openings have been formed by exposure using the photomask shown in FIG. 4A;  
         [0046]    [0046]FIG. 4C is a plan view showing the state of openings that have been caused to shrink by heating the resist shown in FIG. 4B;  
         [0047]    [0047]FIG. 5 is a characteristics chart showing the degree of deformation of the openings caused by heating of the resist;  
         [0048]    [0048]FIG. 6A is a plan view showing the first modification of the photomask according to the present invention;  
         [0049]    [0049]FIG. 6B is a plan view showing the resist in which openings have been formed by exposure using the photomask shown in FIG. 6A;  
         [0050]    [0050]FIG. 6C is a plan view showing the state of openings that have been caused to shrink by heating the resist shown in FIG. 6B;  
         [0051]    [0051]FIG. 7 is a plan view showing an actual example of the dimensions of each part of a photomask according to the present invention;  
         [0052]    [0052]FIG. 8A is a plan view showing a modification of the exposure pattern formed on the resist;  
         [0053]    [0053]FIG. 8B is a plan view showing the pattern of openings that are formed on the photomask of the prior art; and  
         [0054]    [0054]FIG. 8C is a plan view showing the pattern of openings that are formed on a photomask of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0055]    An embodiment according to the present invention will be described below with reference to FIGS.  4 A- 4 C and FIG. 5. Components of this embodiment that are identical to components of the above-described example of the prior art are identified by the same term, and detailed explanation is omitted.  
         [0056]    As shown in FIGS.  4 A- 4 C, photomask  200  of this embodiment is also used to pattern resist  201  in a thermal flow process, and a plurality of exposure openings  202  are formed corresponding to the processing positions of a semiconductor integrated circuit (not shown in the figure) that is to undergo processing.  
         [0057]    In more detail, in the pattern forming method of this embodiment, for example, nine openings  203  arranged in three rows and three columns are formed in resist  201 , which is applied to the surface of semiconductor integrated circuit, as shown in FIG. 4C. The distances between these openings  203  is relatively small in the direction from front to back (vertical direction in the figure) and relatively large in the direction from left to right.  
         [0058]    In the present embodiment as well, nine exposure openings  202  are formed on photomask  200  in three rows and three columns that are close to each other in the front and rear directions but distant from each other toward the right and left, as shown in FIG. 4A, but these exposure openings  202  are formed in a shape that compensates for the anisotropic deformation that occurs in openings  203  when resist  201  is heated to cause openings  203  to shrink.  
         [0059]    In other words, in photomask  200  of this embodiment, each of the plurality of exposure openings  202  that are close to each other in the front and rear directions but distant from each other toward the left and right is formed in a rectangular shape, which is a square that has been enlarged toward the right and left. As a result, in photomask  200  of this embodiment, each of the plurality of exposure openings  202  that are close to each other toward the front and rear and arranged in lines is enlarged toward the left and right, which are directions orthogonal to the direction of arrangement. As a result, these exposure openings  202  are formed as rectangles having long sides that extend in the left and right directions, which are the directions of enlargement, and the sides that extend in the front and rear directions toward the other nearby exposure openings  202  are therefore the directions in which the short sides of these rectangles extend.  
         [0060]    In the pattern forming method of this embodiment, however, each of the plurality of exposure openings  202  of photomask  200  is actually enlarged in almost all directions compared to the dimensions of opening  203  for the exposure process in resist  201 . However, the degree of enlargement of these exposure openings  202  is small in the front and rear directions that extend toward other closely neighboring exposure openings  202 , and exposure openings  202  are thus formed as shapes that are expanded toward the left and right.  
         [0061]    Exposure openings  202 , which are arranged both toward the front and rear and toward the right and left of photomask  200  as described in the foregoing explanation, are also close to each other toward the right and left, although not as close as toward the front and rear, and exposure openings  202  are therefore also enlarged toward the front and rear, which is the direction orthogonal to the right and left. As described above, however, the degree of enlargement of exposure openings  202  is great toward the left and right and small toward the front and rear, the degree of enlargement in each direction being inversely proportional to the distance to a neighboring opening in that direction.  
         [0062]    A brief explanation is next presented regarding a thermal flow process that uses photomask  200  of this embodiment in the above-described construction.  
         [0063]    First, resist  201  is applied to the surface of the semiconductor integrated circuit that is to undergo processing, and this resist  201  is then patterned by means of an exposure process by photomask  200 .  
         [0064]    Then, as shown in FIG. 4B, a plurality of openings  203  corresponding to the plurality of exposure openings  202  of photomask  200  are formed in resist  201 . When this resist  201  is heated and each of openings  203  is caused to shrink, openings  203  of resist  201  attain a small diameter that is less than the exposure limit dimensions, whereby a desired process can be performed in a fine area of a semiconductor integrated circuit.  
         [0065]    When resist  201  is heated and each of openings  203  is caused to shrink as described hereinabove, however, anisotropic deformation occurs according to the positional relationship between these openings  203 . The deformation of these openings  203  is believed to arise due to such factors as the surface tension of resist  201 . When openings  203  are caused to shrink by heating resist  201 , the degree of shrinkage in openings  203  that are close to each other is small in the directions toward the other openings while the degree of shrinking is great in the direction that is orthogonal to this direction, as shown in FIG. 5.  
         [0066]    However, since exposure openings  202  in photomask  200  of this embodiment are formed in shapes that compensate for the anisotropic deformation of openings  203  as shown in FIG. 4A, openings  203  that are formed in resist  201  by an exposure process that uses this photomask  200  are formed in an oval shape that is enlarged in the direction that is substantially orthogonal to the direction toward other openings  203  that are close, as shown in FIG. 4B. When the plurality of openings  203  that are thus formed are caused to shrink by heating resist  201 , openings  203  assume a substantially circular shape as shown in FIG. 4C due to the occurrence of anisotropic deformation according to the positional relation between the openings.  
         [0067]    As a result, fine openings  203  can be formed in a proper shape in resist  201  in a thermal flow process that uses photomask  200  of this embodiment, and appropriate processing can thus be realized in a fine area of semiconductor integrated circuit. In photomask  200  of this embodiment, moreover, each of exposure openings  202  is formed as a rectangle in which the right and left directions, which are the directions of chief enlargement, are the directions in which the long sides extend, and the design and fabrication of of the photomask is thus facilitated.  
         [0068]    The present invention is not limited to the above-described embodiment, and various modifications are possible in the scope or spirit of the invention. For example, in the above-described embodiment, an example was described in which openings  203  of a desired shape were formed on resist  201  by a photolithographic technique using photomask  200 , but these openings  203  may also be formed on resist  201  by a direct writing technique that does not use photomask  200 .  
         [0069]    Although a case was described in the above-described embodiment in which openings  203  that were formed on resist  201  were arranged in lines extending toward the front and rear, openings  203  may also arranged linearly in a direction that is at an angle, as in the previously described example of ¼-pitch DRAM of the prior art. Simply adapting the above-described photomask  200  to this type of arrangement, however, means that the exposure openings that were originally square must be enlarged in a direction that extends at an angle. The exposure openings must therefore be enlarged to form a parallelogram or diamond shape, and this complicates the design and fabrication of the photomask.  
         [0070]    When this becomes a problem, a plurality of exposure openings  211  of photomask  210  are formed in rectangular shapes in which the long sides extend in an oblique direction that is orthogonal to direction of arrangement of these openings  211 .  
         [0071]    In this case, a plurality of openings  212  are formed as inclined oval shapes in resist  201  as shown in FIG. 6B, and these oval openings  212  are enlarged in directions that are orthogonal to the direction in which the openings are close to each other, whereby these openings  212  become proper circles when caused to shrink by heating resist  201 .  
         [0072]    The inventors of the present invention actually produced photomask  210  on an experimental basis in which openings  212  in resist  201  were arranged in a 45° direction as described in the foregoing explanation. In this case, the average diameter of openings  212  in resist  201  that were caused to shrink by heating resist  201  was set to 0.15 μm, and the pitch toward the front and rear as well as to the right and left of the plurality of openings  212  that were arranged in a 45° direction was set to 0.3 μm.  
         [0073]    Taking into consideration the data of FIG. 5, exposure openings  211  of photomask  210  were formed in rectangular shapes with short sides of 0.23 μm and long sides of 0.4 μm as shown in FIG. 7, and it was confirmed that openings  212  were finally formed having substantially the above-described dimensions.  
         [0074]    In the above-described embodiment, an example was described in which regularly arranged openings  203  were formed in resist  201 , but the present invention may also be adapted to a case in which irregularly arranged openings  203  are formed in resist  201 . It is also obvious that the conditions of deformation for openings of identical shape that are arranged in a line at uniform intervals as described hereinabove will differ for openings at the two ends and for openings in the central area.  
         [0075]    However, the inventors designed photomask  222  in which all of openings  221  can be formed in proper shape assuming three openings  221  having a diameter of 0.2 μm are arranged in resist  201  in a line at a pitch of 0.35 μm as shown in FIG. 8A.  
         [0076]    In a photomask of the prior art in such a case, three square exposure openings measuring 0.2 μm on each side are arranged in a line at a pitch of 0.35 μm as shown in FIG. 8B. In contrast, it was confirmed that in photomask  222  of this invention, central exposure openings  223  are preferably formed as rectangles measuring 0.24×0.30 μm and exposure openings  223  at the two ends are preferably formed as rectangles measuring 0.27×0.30 μm, as shown in FIG. 8C. In other words, in a case in which a plurality of exposure openings  223  are arranged in a line, the degree of enlargement in the direction of arrangement of exposure openings  223  at both ends is preferably greater than the degree of enlargement of exposure openings  223  in the central area.  
         [0077]    Among the above-described embodiments, a case was described in which exposure openings  202  of photomask  200  were enlarged in substantially all directions, i.e., toward the front, rear, left and right, with the degree of this enlargement for the front-rear directions differing from that for the left-right directions. However, it is also possible for exposure openings  202  to be enlarged in only specific directions.  
         [0078]    While preferred embodiments of the present invention have been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the following claims.