The effect of laser wavelength on emission and particle dynamics of Sn plasma

Abstract

We investigated the effects of laser wavelength on the atomic, ionic, and radiative emission from laser-produced tin plasmas. For generating plasmas, planar tin targets were excited using either high intensity neodymium-doped yttrium aluminum garnet (Nd:YAG, 1.06 μm) or carbon dioxide (CO2, 10.6 μm) laser pulses; both are considered to be potential excitation lasers for an extreme ultraviolet (EUV) lithography laser-produced plasma light source. Various diagnostic tools were utilized for investigating ionic, neutral, and radiative emission from Sn plasmas including Faraday cup, witness plate in conjunction with x-ray photoelectron spectroscopy (XPS), EUV, and visible emission spectroscopy and photography. Atomic and ionic analysis showed that the amount of debris emitted by the Nd:YAG generated plasmas was considerably higher than the CO2 laser-produced plasmas. The angular distributions of both atomic and ionic debris were found to be more forward-centric for the 1.06 μm generated plasma while being much more uniform for the 10.6 μm heated plasma. EUV and visible emission images of the plasma also showed a forward-centric appearance for 1.06 μm heated plasmas. The strength of excited neutral emission was considerably lower for the case of the 10.6 μm plasma while the kinetic energies of ions debris were found to be much higher for CO2 generated plasmas. Surface analysis of the craters created by the lasers showed that the mass ablation rate is 3.6 times higher for Nd:YAG laser generated plasmas compared to CO2 generated plasmas at maximum EUV emission.