Enhancement of spectral performance in gadolinium-based BEUV sources by supplying pre-formed plasma with cavity-confined targets.

Gadolinium laser-produced plasma (Gd-LPP) represents a promising BEUV lithography light source for future semiconductor manufacturing. In this work, we seek to further enhance the Gd-LPP spectral performance by integrating a cavity-confined target with the existing dual-pulse laser scheme. By regulating the pre-formed plasma with the cavity-confined target, a higher level of spectral purity was achieved in comparison to the flat target, reaching a maximum of 4.35% at a pre-main pulse delay of around 40 ns. Notably, even with extended pre-main pulse delays, the spectral purity of the cavity-confined target drops slowly and remains comparable to the peak spectral purity value achieved by that flat target. Simulations indicate that a gradual density gradient at the ablation front for the cavity-confined target is key to achieving high spectral purity and this is attributed to the fact that the cavity-confined target maintains this gradient when a main pulse laser is irradiated. Our findings offer an approach for enhancing spectral performance by optimizing the pre-formed plasma in a dual pulse scheme.