A method includes following steps. A photoresist-coated substrate is received to an extreme ultraviolet (EUV) tool. An EUV radiation is directed from a radiation source onto the photoresist-coated substrate, wherein the EUV radiation is generated by an excitation laser hitting a plurality of target droplets ejected from a first droplet generator. The first droplet generator is replaced with a second droplet generator at a temperature not lower than about 150° C.