A method including steps as follows is provided. A primary droplet and a satellite droplet are shot toward an excitation zone. The satellite droplet is deflected away from the excitation zone. A laser beam is emitted toward the excitation zone to excite the primary droplet to generate an extreme ultraviolet (EUV) light. The EUV light is directed onto a reticle using a first optical reflector, such that the EUV light is imparted with a pattern of the reticle. The EUV light with the pattern is directed onto a wafer using a second optical reflector.