Tuning extreme ultraviolet emission for optimum coupling with multilayer mirrors for future lithography through control of ionic charge states

We report on the identification of the optimum plasma conditions for a laser-produced plasma source for efficient coupling with multilayer mirrors at 6.x nm for beyond extreme ultraviolet lithography. A small shift to lower energies of the peak emission for Nd:YAG laser-produced gadolinium plasmas was observed with increasing laser power density. Charge-defined emission spectra were observed in electron beam ion trap (EBIT) studies and the charge states responsible identified by use of the flexible atomic code (FAC). The EBIT spectra displayed a larger systematic shift of the peak wavelength of intense emission at 6.x nm to longer wavelengths with increasing ionic charge. This combination of spectra enabled the key ion stage to be confirmed as Gd ¹⁸⁺, over a range of laser power densities, with contributions from Gd¹⁷⁺ and Gd¹⁹⁺ responsible for the slight shift to longer wavelengths in the laser-plasma spectra. The FAC calculation also identified the origin of observed out-of-band emission and the charge states responsible.