Energy and charge distribution of Si ions in EUV ablation plasma

Abstract

Recent developments on high-power compact extreme ultraviolet (EUV or XUV) sources have enabled us to study the materials ablation induced by the irradiation of intense EUV light. The interactions of EUV light with matters along the ablation have potential advantage for the use in advanced materials processing. We have studied the charge states and their energy distributions in the EUV ablation plasma using an E´B mass-charge analyzer. The measurement was also conducted for conventional laser ablation plasma induced by a 1064 nm Nd:YAG laser. The results showed noticeable difference between EUV and laser ablations, where EUV ablation only showed singly charged Si ions, and laser ablation showed multiply charged ions up to Si³⁺. The energy spectra of multiply charged ions were distributed in higher energy range. It has been predicted that EUV ablation plasma have higher electron density close to the solid density and lower electron temperature at 1 to few eV, while laser ablation plasma generally has lower electron density around the critical density and higher electron temperature exceeding 10 eV mainly by inverse bremsstrahlung absorption at an irradiation intensity around 10⁹ W/cm² range. Thus, ionization did not proceed much and only singly charged ions were detected for EUV ablation and the electron impact ionization proceeded to have multiply charged ions for laser ablation. This study showed that the experimental results reflected the general characteristics of the ablation plasma, and showed possibilities of modeling of entire EUV ablation dynamics from expanding ions.