Comparative study on EUV and debris emission from CO₂ and Nd: YAG laser-produced tin plasmas

The emission characteristics of debris from laser-produced tin plasma were investigated for an extreme ultraviolet lithography (EUV) light source. The ions and droplets emitted from tin plasma produced by a CO₂ laser or an Nd: YAG laser were detected with Faraday cups and quartz crystal micro-balance (QCM) detectors, respectively. A higher ion kinetic energy and a lower droplet emission were observed in the case of CO₂ laser in compared with Nd: YAG laser for the same laser energy (50 mJ). The reason comes from the interaction of the laser pulse with the plasma. CO₂ laser is absorbed in superficial region of the plasma due to the long wavelength, in contrast, Nd: YAG laser penetrates deeply into the plasma. Therefore, the absorbed energy density of CO₂ laser will be much larger than that of Nd: YAG laser for the same laser energy. This causes the higher ion energy and complete vaporizing the target material.