1. A method, comprising: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., wherein refilling the reservoir of the droplet generator is performed in a vacuum environment.