A method includes ejecting a metal droplet from a reservoir of a droplet generator toward a zone of excitation in front of a collector, emitting an excitation laser toward the zone of excitation, such that the metal droplet is heated by the excitation laser to generate extreme ultraviolet (EUV) radiation, halting the emission of the excitation laser, depressurizing the reservoir of the droplet generator, cooling down the droplet generator to a temperature not lower than about 150° C., and refilling the reservoir of the droplet generator with a solid metal material at the temperature not lower than about 150° C.