Efficient generation of extreme ultraviolet light from Nd:YAG-driven microdroplet-tin plasma

We experimentally investigate the emission of extreme ultraviolet (EUV) light from a mass-limited laser-produced plasma over a wide parameter range by varying the diameter of the targeted tin microdroplets and the pulse duration and energy of the 1-μm-wavelength Nd:YAG drive laser. Combining spectroscopic data with absolute measurements of the emission into the bandwidth around relevant for nanolithographic applications, the plasma’s efficiency in radiating EUV light is quantified. All observed dependencies of this radiative efficiency on the experimental parameters are successfully captured in a geometrical model featuring the plasma absorption length as the primary parameter. It is found that laser intensity is the pertinent parameter setting the plasma temperature and the tin ion charge state distribution when varying laser pulse energy and duration over almost two orders of magnitude. These insights enabled to obtain a record-high conversion efficiency of laser light into radiation and to identify paths towards obtaining even higher efficiencies with solid-state lasers that may rival those of current state-of-the-art CO2-laser-driven sources.

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