Properties of the extreme ultraviolet emission from germanium and gallium plasmas

We report on the usefulness of germanium (Ge) and gallium (Ga) plasmas as potential extreme ultraviolet (EUV) sources at both 6.x and 13.5 nm, wavelengths of interest for lithography and metrology applications. Spectra from plasmas produced using neodymium-doped yttrium-aluminum-garnet (Nd:YAG) lasers with pulse durations of 170 ps and 7 ns [full width at half-maximum] and a compact electron beam ion trap have been used to determine the charge states responsible while theoretical calculations using atomic structure Cowan and HULLAC codes enabled the identification of structure in the Ge spectrum arising from strong transitions of the type 3d – 4p and 3d – 4f. Transitions of the type 3d – 4f in Ge¹⁰⁺ and Ga¹¹⁺ were found to lie in the 6.x nm region, while $3p−3d$ transitions from a range of stages in both elements contribute in the 13.5 nm region. We also studied the emission spectra of galinstan plasmas, and the results imply that galinstan might provide a promising candidate for EUV applications that exploit the progress in the development of multilayer mirrors for operation at 6.x nm.