A method of forming a polycrystalline silicon layer includes: disposing a mask over the amorphous silicon layer, the mask having a plurality of transmissive regions, the plurality of transmissive regions being disposed in a stairstep arrangement spaced apart from each other in a first direction and a second direction substantially perpendicular from the first direction, each transmissive region having a central portion and first and second side portions that are adjacent to opposite ends of the central portion along the first direction, and wherein each of the portions has a length along the first direction and a width along the second direction, and wherein the width of first and second portions decreases away from the central portion along the first direction; irradiating a laser beam onto the amorphous silicon layer a first time through the mask to form a plurality of first irradiated regions corresponding to the plurality of transmissive regions, each first irradiated region having a central portion, and first and second side portions at both sides of the central portion; moving the substrate and the mask relative to one another such that the first side portion of each transmissive region overlaps the central portion of each first irradiated region; and irradiating the laser beam onto the amorphous silicon layer a second time through the mask to form a plurality of second irradiated regions corresponding to the plurality of transmissive regions.