System and method for controlling droplet timing in an LPP EUV light source

A method and apparatus for improved control of the trajectory and timing of droplets of target material in a laser produced plasma (LPP) extreme ultraviolet (EUV) light system is disclosed. A droplet illumination module generates two laser curtains for detecting the droplets. The first curtain is used for detecting the position of the droplets relative to a desired trajectory to the irradiation site so that the position of a droplet generator may be adjusted to direct the droplets to the irradiation site, as in the prior art. A droplet detection module detects each droplet as it passes through the second curtain, determines when the source laser should generate a pulse so that the pulse arrives at the irradiation site at the same time as the droplet, and sends a signal to the source laser to fire at the correct time.