Interaction of a CO2 Laser Pulse With Tin-Based Plasma for an Extreme Ultraviolet Lithography Source

The interaction of a CO2 laser pulse with Sn-based plasma for a 13.5-nm extreme ultraviolet (EUV) lithography source was investigated. It was noted that a CO2 laser with wavelength of 10.6 μm is more sensitive to surface impurities as compared with a Nd:YAG laser with wavelength of 1.06 μm. This reveals that a CO2 laser is more likely absorbed in a thinner layer near the target surface. Compared with a Nd:YAG laser, a CO2 laser shows higher in-band (2% bandwidth) conversion efficiency (CE) with a solid Sn target due to less reabsorption of the EUV emission induced by the plasma. However, with foam targets containing low concentrations of Sn, the in-band CE is lower than that with solid Sn. The CE can be enhanced with plasma confinement. These results suggest that a driving laser with wavelength between 1.06 and 10.6 μm may be an even better choice to generate higher CE from laser to 13.5-nm EUV emission.