To improve the laser annealing process for polycrystallizing amorphous silicon to form silicon thin films having large crystal particle diameters at a high throughput, the present invention is directed to a process of crystallization by irradiation of a semiconductor thin film formed on a substrate with pulsed laser light. The process comprises having a means to shape laser light into a linear beam and a means to periodically and spatially modulate the intensity of pulsed laser in the direction of the long axis of the linear beam by passing through a phase-shifting stripy pattern perpendicular to the long axis, and collectively forming for each shot a polycrystalline film composed of crystals which have grown in a certain direction over the entire region irradiated with the linear beam.
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