A wide-slit lateral growth projection mask, projection system, and corresponding crystallization process are provided. The mask includes an opaque region with at least one a transparent slit in the opaque region. The slit has a width in the range of 10X to 50X micrometers, with respect to a X:1 demagnification system, and a triangular-shaped slit end. The triangular-shaped slit end has a triangle height and an aspect ratio in the range of 0.5 to 5. The aspect ratio is defined as triangle height/slit width. In some aspects, the triangular-shaped slit end includes one or more opaque blocking features. In another aspect, the triangular-shaped slit end has stepped-shaped sides. The overall effect of the mask is to promote uniformly oriented grain boundaries, even in the film areas annealed under the slit ends.