Ablation dynamics of tin micro-droplet irradiated by double pulse laser used for extreme ultraviolet lithography source

The ablation dynamics of a tin (Sn) micro-droplet by double pulse laser irradiation for the development of an extreme ultraviolet lithography source was investigated. The solid state Sn droplet target with a diameter of 30 νm was irradiated by double laser pulses from a Q-switched Nd : YAG laser and a CO₂ laser, and the kinetic behaviour of debris such as Sn atoms, ions and of the dense particles from the droplet was investigated by the laser-induced fluorescence imaging method, Faraday cups and high-speed imaging, respectively. After the pre-pulse irradiation of the Nd : YAG laser, the ions were emitted towards the laser incident direction with an average kinetic energy of 3-6 keV and the dense particle cloud moved in the direction opposite to the laser incident direction with expansion by a reaction force due to the plasma expansion at a speed of approximately 500 m s^-1. On the other hand, the Sn atoms were ejected in all directions from the target with a speed as fast as 20 km s^-1. The expanding target was subsequently irradiated by the main pulse of the CO₂ laser with a delay of 800 ns and the dense cloud almost disappeared due to main-pulse irradiation.