Method of suppressing energy spikes of a partially-coherent beam

#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.

#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. 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.