A method includes shooting a primary droplet and a satellite droplet from a droplet generator along a common initial direction; applying a force to the primary droplet and the satellite droplet, wherein after applying the force, the primary droplet has a first deflection toward a first direction different than the common initial direction, and the satellite droplet has a second deflection toward a second direction different than the common initial direction, wherein the second deflection of the satellite droplet is greater than the first deflection of the primary droplet; and generating an extreme ultraviolet (EUV) light using an excitation laser hitting the primary droplet with the first deflection.