A method includes ejecting a metal droplet from a reservoir of a first droplet generator assembled to a vessel; emitting an excitation laser from a laser source to the metal droplet to generate extreme ultraviolet (EUV) radiation; turning off the first droplet generator; cooling down the first droplet generator to a temperature not lower than about 150° C.; dismantling the first droplet generator from the vessel at the temperature not lower than about 150° C.; and assembling a second droplet generator to the vessel.