On the maximum conversion efficiency into the 13.5-nm extreme ultraviolet emission under a steady-state laser ablation of tin microspheres

Abstract

Theoretical investigation has been performed on the conversion efficiency (CE) into the 13.5-nm extreme ultraviolet (EUV) radiation in a scheme where spherical microspheres of tin (Sn) are simultaneously irradiated by two laser pulses with substantially different wavelengths. The low-intensity short-wavelength pulse is used to control the rate of mass ablation and the size of the EUV source, while the high-intensity long-wavelength pulse provides efficient generation of the EUV light at λ=13.5 nm. The problem of full optimization for maximizing the CE is formulated and solved numerically by performing two-dimensional radiation-hydrodynamics simulations with the RALEF-2D code under the conditions of steady-state laser illumination. It is shown that, within the implemented theoretical model, steady-state CE values approaching 9% are feasible; in a transient peak, the maximum instantaneous CE of 11.5% was calculated for the optimized laser-target configuration. The physical factors, bringing down the fully optimized steady-state CE to about one half of the absolute theoretical maximum of CE≈20% for the uniform static Sn plasma, are analyzed in detail.

Efficient Generation of Extreme Ultraviolet Light From Nd :YAG-Driven Microdroplet-Tin Plasma

“…This limit value is lowered by considering that light is also emitted in the opposite, forward-facing half sphere. Indeed, a more stringent limit of CE ≤ SP/2 holds in the case of spherically symmetric emission [54]. These limits emphasize the importance of spectroscopic investigations as introduced here.…”

“…We note that the ever more slowly decreasing values for SP/2 and increasing CE appear to converge to a common value. This intriguing observation indicates that the limit CE ≤ SP/2, strictly valid only for isotropic emission [54], may provide a good estimate for the maximally reachable CE value of the plasma source.…”

Power-law scaling of plasma pressure on laser-ablated tin microdroplets

“…Radiation generation and transport is treated with the same opacity model as in Ref. 24, where the conversion efficiency into the 13.5nm EUV emission is investigated for a CO 2 -laser-driven plasma. The angular dependence of the radiation inten-sity is modeled with the S 6 quadrature, while the spectral dependence is simulated with 28 discrete spectral groups of variable width.…”

Prominent radiative contributions from multiply-excited states in laser-produced tin plasma for nanolithography

“…In the context of understanding and supporting the drive laser wavelength change in future industrial sources, calculation of complete and accurate opacity spectra and of the atomic data therein is essential for predictive simulations of source performance. These data are needed in radiation hydrodynamics codes [23][24][25][26] and for the calculation of emission spectra. Without such accurate opacity data, the capability for predictive modeling would be severely impaired as, for example, modest underestimations of opacity could lead to significant underestimation of required drive laser intensities.…”