A novel two-pass excimer laser crystallization process to obtain homogeneous large grain polysilicon

New approach to control the lateral growth mechanism through the opportune spatial modulation of the absorbed laser energy and with a two-pass excimer laser crystallization process is presented. In the first pass, spatial modulation of the light intensity has been obtained by irradiating the sample through a patterned mask in contact with the sample. Lateral growth is triggered when the irradiated regions are fully melted and a lateral extension of the grains in excess to 1 ?m has been observed for samples irradiated at RT. In order to homogeneously crystallize the sample, the film can be re-irradiated (second pass) without the mask. By using opportune energy densities it can be induced a complete melting of the residual a-Si regions (masked areas during the first pass), while partially melting the polysilicon regions (unmasked areas during the first pass). Different mask geometries have been investigated and for optimized conditions, the sample area can be fully covered with laterally grown grains. The proposed novel technique can be rather attractive for polysilicon TFT fabrication, being characterized by only a two laser-shot process and wide energy density windows.