Dynamics of laser-produced Sn-based plasmas for a monochromatic 13.5 nm extreme ultraviolet source

Dynamics of laser-produced Sn-based plasmas were investigated for a monochromatic EUV lithography (EUVL) source. A hollow plasma density in a Sri plasma driven by Nd:YAG laser was observed in the late time within the laser pulse. The possible reason comes from the distributed laser energy deposition in the expanding corona. This distributed absorption results in a temperature gradient in the corona and a broad EUV spectrum. It was shown that for CO, laser most of the laser energy deposition is localized around the critical density, a narrower EUV x-ray spectrum and a higher conversion efficiency from laser to monochromatic 13.5 nm EUV emission can be expected. It was found that 0.5% Sn-doped foam targets show an almost the same electron density as compared with that of solid density Sri targets. The same n(e) enables efficient absorption of laser energy, and at the same time much lower Sn ion number density results in less re-absorption of the in-band 13.5 nm EUV emission induced by the plasma itself, so high CE can be expected with a low concentration of Sn.