Abstract:
Accordingly, a method of suppressing energy spikes is provided comprising projecting a laser beam through a mask having a slit pattern comprising a corner region with edges, and a blocking feature with the corner region to reduce energy spikes projected on a substrate. An alternative method is provided, wherein the corner region is modified such that it is replaced by a more tapered shaped region, preferably a triangle. Also provided, are a variety of mask designs incorporating both corner regions, with and without one or more blocking features, and triangular regions, with or without one or more blocking features. The mask designs provide examples of mask modifications that may be used to reduce energy spikes.

Description:
CROSS-REFERENCE 
   This application is a divisional of application Ser. No. 10/113,144, filed Mar. 27, 2002, entitled “Method for Suppressing Energy Spikes of a Partially-Coherent Beam,” invented by Apostolos T. Voutsas, Mark Albert Crowder, and Yasuhiro Mitani now U.S. Patent No. 6,792,029, which is hereby incorporated by reference. 

   BACKGROUND OF THE INVENTION 
   A laser beam is used in laser-annealing applications to crystallize an amorphous material to obtain a crystalline, or polycrystalline, material. For example, excimer lasers may be used to crystallize an amorphous silicon (a-Si) film to obtain a polycrystalline silicon (poly-Si) region. In several of these methods, a mask is inserted into the path of the beam to shape the laser beam before the beam irradiates the material. 
   The mask, used to shape the beam, can in principal have a wide variety of patterns on it. The mask may comprise a patterned layer of chrome, or other material that blocks the desired wavelength effectively, on a quartz substrate, or other suitably transparent material at the wavelength of laser to be used. Common patterns consist of groups of rectangular shapes, including slits and chevrons. 
   As the laser beam is projected through these patterns, the intensity profile of the projected beam will be determined by the features that make up the pattern and any optics used to image the pattern on the material. The intensity profile of the laser beam is typically not uniform over the entire pattern. For example, at corner regions intensity peaks have been noticed. These intensity peaks may cause local damage on the film irradiated by the shaped beam. One form of damage caused by intensity peaks is agglomeration, which may cause the silicon film to pull away from the region exposed to the high intensity peaks, possibly leaving a void or other non-uniformity. 
   SUMMARY OF THE INVENTION 
   Accordingly, a method of suppressing energy spikes is provided comprising projecting a laser beam through a mask having a slit pattern comprising a corner region with edges, and a blocking feature with the corner region to reduce energy spikes projected on a substrate. An alternative method is provided, wherein the corner region is modified such that it is replaced by a more tapered shaped region, preferably a triangle. 
   Also provided, are a variety of mask designs incorporating both corner regions, with and without one or more blocking features, and triangular regions, with or without one or more blocking features. The mask designs provide examples of mask modifications that may be used to reduce energy spikes. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a plane view showing two prior art mask patterns 
       FIG. 2  is a plane view showing a portion of a mask pattern. 
       FIG. 3  is a 3D intensity distribution plot corresponding to the mask pattern of  FIG. 2 . 
       FIG. 4  is a plane view showing a portion of a mask pattern. 
       FIG. 5  is a 3D intensity distribution plot corresponding to the mask pattern of  FIG. 4 . 
       FIG. 6  is a plane view showing a portion of a mask pattern. 
       FIG. 7  is a 3D intensity distribution plot corresponding to the mask pattern of  FIG. 6 . 
       FIG. 8  is a plane view showing a portion of a mask pattern. 
       FIG. 9  is a 3D intensity distribution plot corresponding to the mask pattern of  FIG. 8 . 
       FIG. 10  is a plane view showing a portion of a mask pattern 
       FIG. 11  is a 3D intensity distribution plot corresponding to the mask pattern of  FIG. 10 . 
       FIG. 12  is a plane view showing a portion of a mask pattern. 
       FIG. 13  is a 3D intensity distribution plot corresponding to the mask pattern of  FIG. 12 . 
       FIG. 14  is a plane view showing a portion of a mask pattern. 
       FIG. 15  is a 3D intensity distribution plot corresponding to the mask pattern of  FIG. 14 . 
       FIG. 16  is a plane view showing a portion of a mask pattern. 
       FIG. 17  is a 3D intensity distribution plot corresponding to the mask pattern of  FIG. 16 . 
       FIG. 18  is a plot of the critical dimension of blocking features related to the numerical aperture of the projection system. 
       FIG. 19  shows a region following laser annealing using a mask similar to that shown in  FIG. 6 . 
       FIG. 20  shows a region following a two-pass laser annealing process using masks similar to that shown in  FIG. 6 . 
       FIG. 21  is a plane view showing a portion of a mask pattern. 
       FIG. 22  shows a region following laser annealing using a mask similar to that shown in  FIG. 21 . 
       FIG. 23  shows a region following a two-pass laser annealing process using masks similar to that shown in  FIG. 21 . 
       FIG. 24  is a plane view of a portion of a mask pattern similar to that shown in  FIG. 21  with the addition of blocking features. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  shows a mask  8  with two examples of common mask patterns used in connection with laser annealing processes. A slit  10  has a first corner region  12  and a second corner region  14 , as indicated by the dotted ellipses. A chevron  16  also has corner regions  18  and  20 , as indicated by the dotted ellipses. It is in the vicinity of corner regions, that intensity spikes have been notice when a laser beam is projected through the mask  8  onto a substrate to be laser annealed. The laser beam may pass through a variety of projection optics either before or after passing through the mask  8 . 
     FIG. 2  illustrates a portion  30  of a prior art mask pattern for use in laser annealing processes. The portion  30  includes a slit  32  with a corner region  34 . The slit  32  is essentially transparent at the desired wavelength across its entire area. Transparent refers to an area with no opaque pattern within the slit, even through there may be slight attenuation from the quartz or other substrate that the pattern has been formed on. The slit  32  as shown has a width of between approximately 4 and 5 micrometers wide. However, the width of the slit is only limited by the lateral crystal growth length. For example, the slit width could be in the range of between approximately 2 and 20 micrometers. 
     FIG. 3  is a laser intensity distribution plot for the slit  32  shown in  FIG. 2 . The intensity distribution depends upon the numerical aperture (NA) of any projection system used to image the laser beam that passes through the slit  32  onto the material to be crystallized. The intensity distribution shown in  FIG. 3  is based on a projection system with a numerical aperture of 0.13. The intensity distribution becomes more intense in the region at the end of the corner region. An intensity peak  40  is clearly visible on this intensity plot. The intensity peak  40  produces undesirable material uniformity effects. The problems associated with the intensity peak  40  are further compounded when scanning laser annealing processes are used as the corner regions may-pass over areas of material that have already been exposed to corner regions from previous passes. This may contribute to additional damage from successive intensity peaks passing over the same region of material. Elimination of the intensity peak will reduce, or eliminate, damage to the crystallized materials, including agglomeration, and other material non-uniformity effects, caused by the intensity peak. 
   Therefore, it would be desirable to have a mask design and corresponding process that suppresses the corner effect and allows for uniform, crystallized material to be generated upon laser irradiation. 
   Several families of designs were considered to modify the corner region to reduce, or eliminate, the intensity peak. A first family of designs maintains the outline of the slit, but incorporates beam blocking features in the vicinity of the corner region. A second family of designs modified the outline of the slit to form a triangular region in place of the corner region, both with and without beam blocking features. Another family of designs modified the outline of the slit by using a stepped triangular region in place of the corner region, along with the possible addition of blocking features. 
   Examples 1 through 7 are based on rectangular slits that are either four, or four and a half, micrometers wide projected onto a material using a projection system with a numerical aperture of 0.13 so that the resulting intensity distributions can be compared with those corresponding to an unmodified slit as described in connection with  FIGS. 2 and 3 . 
   The dimensions provided here are illustrative as the critical dimensions and spacing of blocking features depend on the projection system to be used. 
   EXAMPLE 1 
   Referring now to  FIG. 4 , a portion of a mask  50  is shown. The mask  50  has a slit  52  with a blocking feature  54 . For example, the slit  52  has an end  56  along with a first side  58  and an opposite side  60 . The slit is four micrometers wide measured from the first side  58  to the opposite side  60 . The blocking feature  54 , as shown, is one micrometer wide, across its narrowest dimension, one micrometer from the end  56 , and one micrometer from either side  58  or  60 . 
     FIG. 5  shows an intensity distribution corresponding to the mask  50 , shown in  FIG. 4 . The intensity peak  40  has been shifted away from the end of the slit. A secondary peak  42  is also shown. The secondary peak  42  is well below the baseline intensity, shown at  44 . By shifting the intensity peak  40  away from the end of the slit, it may be possible to reduce material damage associated with successive scan exposures, since there is less chance of the intensity peaks overlapping. However, the intensity peak may still damage crystallized material during a single pass. 
   EXAMPLE 2 
     FIG. 6  shows another example of the mask  50  with the blocking feature  54  positioned toward the end  56  of a slit  52 . As shown the slit  52  is 4 micrometers wide. The blocking feature  54  is approximately one half micrometer wide and positioned approximately one half micrometer from the end  56  and from either side  58  or  60 . 
     FIG. 7  shows an intensity distribution corresponding to the mask  50 , shown in  FIG. 6 . The intensity peak  40  has been shifted slightly away from the end of the slit. The intensity peak  40  is significantly reduced, although still identifiable. 
   EXAMPLE 3 
     FIG. 8  shows another example of the mask  50 . In this example multiple blocking features  54  are used. As shown, the slit  52  is 4.5 micrometers wide. The blocking features  54  are squares with sides approximately one half micrometer long. Each blocking feature is separated from the next by a half micrometer space. There is also a half micrometer space from the end of the slit  52  to the blocking features, as well as between either side  58  and  60  and the nearest of the blocking features  54 . 
     FIG. 9  shows an intensity distribution corresponding to the mask  50 , as shown in  FIG. 8 . There is no longer an identifiable intensity peak. The intensity distribution is essentially flat across the slit region. The edge of the intensity distribution appears to have shifted only slightly from the end of the slit. 
   EXAMPLE 4 
     FIG. 10  provides another example of the mask  50 . The corner region of the slit  52  has been modified. The rectangular corner region has been replaced by a triangular end region  64 . The triangular end region may be patterned, as shown, so that the triangular end region corresponds to a triangle having an apex  66  at the position corresponding to the middle of the end of the corner region of the corresponding rectangular slit. As shown the width of the rectangular region is approximately four micrometers, which corresponds to the base of the triangular region. 
     FIG. 11  shows an intensity distribution corresponding to the mask  50 , shown in  FIG. 10 . The intensity peak  40  has essentially the same magnitude at the peak, as the intensity peak shown in  FIG. 3 . The intensity peak  40  is not as wide, however, as it tapers off at an angle along the edges corresponding to the sides of the triangular region of the slit  52 . 
   EXAMPLE 5 
   A modified version of Example 4 is provided in  FIG. 12 . Blocking features  70 ,  72 , and  74  have been added to the triangular end region. The blocking features, as shown, consist of two small triangles  70  and  72 , which are right triangles with sides that are approximately one half micrometer each. Each small triangle  70  and  72  is positioned such that one of its sides is approximately one half micrometer from the edge of the triangular end region of the slit  52 . The side may be substantially parallel to the slit edge, as shown. A third blocking feature  74  is a right triangle with a hypotenuse that is one micrometer long. The third blocking feature  74  has a right angle positioned approximately one half micrometer from the apex  66  of the triangular region. 
     FIG. 13  shows an intensity distribution corresponding to the mask  50 , shown in  FIG. 12 . The intensity peak  40  has been significantly reduced, although still identifiable. The intensity distribution is sufficiently flat for purposes of laser crystallization processes, and should not induce significant damage. 
   EXAMPLE 6 
   Further modification of Example 5, is shown in  FIG. 14 . A rectangular blocking feature  76  has been added adjacent to the three triangular blocking features  70 ,  72 , and  74 , such that it is approximately one half micrometer from each of the triangular blocking features. As shown, the rectangular blocking feature is approximately one half micrometer wide and one micrometer in length. 
     FIG. 15  shows an intensity distribution corresponding to the mask  50 , shown in  FIG. 14 . There are now two intensity peaks  41  and  43  identifiable. The intensity distribution is effectively flat for purposes of laser crystallization processes, and should not induce significant damage. 
   EXAMPLE 7 
     FIG. 16  shows another illustration of a modified corner region of a slit  52  on a mask  50 . The corner region has been replaced by a stepped triangular feature. As shown, the triangular region has a tip region  80 , which is one micrometer across at its apex  86 , with square steps each having a one half micrometer rise and one half micrometer run. The width of the rectangular region of the slit  52 , as shown, is four micrometers wide. 
     FIG. 17  shows an intensity distribution corresponding to the mask  50 , shown in  FIG. 16 . The intensity peak  40  has essentially the same magnitude at the peak as that shown in  FIG. 3 . The intensity peak  40  is not as wide, however, as it tapers off at an angle along the edges corresponding to the sides of the triangular region of the slit  52 . The intensity distribution appears to correspond very closely with that of  FIG. 11 , which corresponds to a triangular region. It is anticipated that the addition of blocking features would reduce the intensity peak. Either rectangular, square, triangular or other appropriate shapes could be used as blocking features. One or more blocking features could be added. Based upon the results of shown in  FIGS. 7 ,  9 ,  13  and  15 , blocking features approximately one half micrometer across may be appropriate. 
   The above results suggest that the addition of blocking features, in the vicinity of the corners, is one way to obtain a uniform intensity profile. The critical dimension of the blocking features depend primarily upon the resolution of the optical system that is used to image the mask onto the substrate. The resolution depends upon the numerical aperture (NA) of the optical system. The higher the NA; the higher the resolution will be (i.e. the optical system will be able to resolve finer features). In the intensity distributions shown above, a numerical aperture of 0.13 was used, corresponding to a resolution of approximately 1.6 to 2 micrometers. For this NA, the critical blocking feature dimension that optimizes the intensity profile is on the order of 0.35 to 0.7 micrometers. Further optimization may narrow the critical blocking feature dimension range to between approximately 0.4 and 0.6 micrometers, and as discussed above one half micrometer may be optimal. 
   As the NA of the optical system changes, the optimum blocking feature dimension-range will also change. Therefore, we can plot the optimum block dimension-range as a function of the NA of the imaging system. This plot is shown in  FIG. 18 , which shows the critical dimension of blocking features in micrometers as a function of the numerical aperture (NA) of the projection lens. The examples given above corresponded to an optical system with an NA of 0.13. The plot shows the critical dimensions of blocking features for projections lenses having both larger and smaller numerical apertures, than the NA of 0.13 used in the above examples. 
   Depending on the NA of the optical system, the critical dimension for the size, or width, of blocking features and the spaces between features and other portions of the slit will fall within a range. Although for ease of illustration a single value has been used for the both the blocking feature size and the space between blocking features in the above examples, it is possible for the sizes of the blocking features to be different from each other, and that the spaces between blocking features be different sizes than the features themselves. The term critical dimension corresponds to a range of sizes, and the features and spaces may have varying sizes, preferably the sizes, or widths, will be within the critical dimension range. 
   Referring now to  FIG. 19 , a polycrystalline region  100  is shown surrounded by an amorphous region  102 . The polycrystalline region  100  was formed using a single laser pulse through a mask similar to that described in connection with  FIG. 6 . The use of the mask of  FIG. 6  with a blocking feature has eliminated agglomeration. However, another unexpected crystalline non-uniformity is identifiable. In the end region identified at  104 , a radial crystal pattern is clearly visible. The crystal pattern away from the end region is a more consistent, substantially parallel pattern. 
   Referring now to  FIG. 20 , a polycrystalline region  110  is shown after completing a multi-pass laser annealing scan using the same mask as that used in connection with  FIG. 19 . The region  114 , which corresponds to overlapping end regions, still shows the less desirable radial crystal pattern. Although, a radial crystal pattern is preferable to damage such as that caused by agglomeration, in a preferred embodiment it would be desirable to also reduce, or eliminate, the radial crystal regions. 
     FIG. 21  illustrates a mask pattern  116  with a triangular end region  118 . The triangular end region  118  has a base  120  and a length  122 , that form the basis of the aspect ratio of the triangular end region. The aspect ratio is the ratio of the length  122  to the base  120 . The aspect ratio of the triangular end region is in the range between one half and five. One preferred range of aspect ratios is between two and four. The aspect ratio of the embodiment shown in  FIG. 21  is approximately three. 
     FIG. 22  shows a polycrystalline region  200  surrounded by an amorphous region  202 . The polycrystalline region  200  was formed using a single laser pulse through a mask similar to that described in connection with  FIG. 21 . In the end region identified at  204 , the radial crystal pattern has been reduced or eliminated. On close inspection of the end region identified at  204 , a slight fishbone pattern can be seen. This fishbone pattern is closer to the more desirable substantially parallel pattern found outside the end region. 
   Referring now to  FIG. 23 , a polycrystalline region  210  is shown after completing a multi-pass laser annealing scan using the same mask as that used in connection with  FIG. 22 . The region  214 , which corresponds to the overlapping end region, no longer shows a radial crystal pattern. In addition, there is not even a clearly discernable fishbone crystal pattern. The resulting material appears to be substantially parallel throughout the crystallized region. 
     FIG. 24  illustrates a mask pattern  216  which has a triangular end region similar to that shown in  FIG. 21 . Additional blocking features  218  have been added to provide additional control over beam intensity to further reduce, or eliminate, energy spikes. Although  FIG. 24 , as well as  FIG. 21 , are shown as straight-sided triangles, it would also be possible to use stepped triangles instead.