A comparison of the emissions, densities, temperatures, and debris features of laser-produced plasmas

Tin and lithium plasmas emit efficiently in the in-band region (13.5 nm with 2% bandwidth) necessary for extreme ultraviolet (EUV) lithography. A detailed comparison of the atomic and ionic debris, as well as the emission features of tin and lithium plasmas has been conducted under identical experimental conditions. Planar slabs of pure tin and lithium were irradiated with 1064 nm, 9 ns neodymium-doped yttrium aluminum garnet (Nd:YAG) laser pulses for producing plasmas. A suite of diagnostics were used to analyze the emission and debris features, including optical emission spectroscopy (OES), a Faraday cup, an EUV pinhole camera, a power tool for the absolute measurement of EUV conversion efficiency (CE), etc. The results show that tin plasmas provide a CE nearly twice that of lithium. However, the kinetic energies of tin ions are considerably higher, though with a lower flux. OES studies have shown that the kinetic energies of neutral species are substantially lower compared to that of the charged particle species. Laser-produced lithium plasmas were also studied in closer detail with 2D OES, conventional 1D OES to determine the electron density throughout the entire plasma from the Stark broadening of the spectral line emissions, along with Nomarski interferometry. The electron temperatures of both neutral and singly-ionized species were estimated through a comparison of the line intensities by using the Boltzmann plot method. These results were then compared to previous studies of tin plasmas under similar conditions. Lithium plasma density was also evaluated using 2D OES in air, argon, and helium buffer gas environments of different pressures. These studies were then repeated along several planes perpendicular to the target surface, to evaluate any radial dependence on lithium plasma electron density