Laser irradiation apparatus, laser irradiation method, and method for manufacturing crystalline semiconductor film

It is an object of the present invention to provide a compact laser irradiation apparatus and a laser irradiation method that use a galvanometer mirror and an fθ lens, that suppress the interference caused by a secondary beam reflected on the rear surface of the substrate, that can perform homogeneous laser annealing to the irradiated object, and that increase throughput. In the present invention, the laser beam has a pulse width t that satisfies an inequality ct<2nd where c is a speed of light in vacuum, n is the refractive index of a substrate with the processing object formed thereon, and d is the thickness of the substrate. With the above structure, even when the galvanometer mirror and the fθ lens are used, it is possible to decrease the effect of the interference due to the secondary beam reflected on the rear surface of the substrate and to perform homogeneous laser annealing to the processing object.