Dynamics of debris from laser-irradiated Sn droplet for EUV lithography light source

In the development of extreme ultraviolet (EUV) light source at 13.5 nm for EUV lithography system by laser-produced plasma (LPP), a Tin (Sn) micro-droplet target is considered as a one of the promising targets for debris mitigation. Inaddition, double pulse irradiation scheme is regarded to be effective in order to improve the conversion efficiency to EUV light in the use of the droplet target. In our study, the dynamics of debris from the Sn droplet target irradiated bydouble pulses was investigated in order to establish the guideline for the optimum design of the mitigation system. The kinetic behaviors of the Sn atoms and of the dense particles from Sn droplet target irradiated by double pulses from the Nd:YAG laser and the CO₂ laser were investigated by the laser-induced fluorescence imaging method and a high-speed imaging, respectively. After the pre-pulse irradiation of the Nd:YAG laser, the Sn atoms were ejected in all direction from the target with a speed of as fast as 20 km/s and the dense particle cloud expanded by a reaction force due to the plasma expansion with a speed of approximately 500 m/s. The expanding target was subsequently irradiated by the mainpulse of CO₂ laser and the dense cloud was almost disappeared by main-pulse irradiation.