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.