Material and Intensity Dependence of EUV Conversion Efficiency in Laser-Driven High-Z Slabs
Year: 2026
Type: conference-paper
Abstract: There is significant effort to find a plasma source with a wavelength shorter than 13.5 nm for nextgeneration semiconductor lithography${ }^{1,2}$. Gadolinium (Gd) and bismuth (Bi) laser-produced plasmas are promising beyond-extreme-ultraviolet (BEUV) light sources. This study identifies optimal plasma conditions for efficient BEUV generation from Gd and Bi, and investigates the influence of laser intensity and detector geometry on spectral output and conversion efficiency. Tin (Sn) plasmas emitting at 13.5 nm are included as a reference EUV source to benchmark plasma dynamics and radiation characteristics. Radiation-hydrodynamic simulations were performed using the FLASH code and post-processed with SPECT3D to compute emission spectra for laser intensities of$\sim 10^{11}-10^{12} ~\mathrm{W} / \text{cm}^{2}$, employing a 1064 nm, 10 ns Gaussian pulse with a$100 \mu m$focal spot. The temporal and spatial evolution of the plasmas was analyzed for all targets. Sn produced strong in-band 13.5 nm emission at$2.4 \times 10^{11} ~\mathrm{W} / \text{cm}^{2}$from charge states$\text{Sn}^{8+}-\text{Sn}^{14+}$at electron temperatures between 20 and 40 eV, while lower intensities suppressed in-band emission and shifted the peak to longer wavelengths. Gd generated intense 6.7 nm emission at (3-9)$\times 10^{11} \mathrm{W} / \text{cm}^{2}$from$\text{Gd}^{12+}-\text{Gd}^{25+}(\mathrm{T}_{\mathrm{e}} \text{up to} \sim{}100 \text{eV})$, with a slight blueshift at higher intensities. Bismuth required higher intensities$\left(6 \times 10^{11}-1.2 \times 10^{12} ~\mathrm{W} / \text{cm}^{2}\right)$to produce strong 3.5-5 nm emission from$\text{Bi}^{25+}-\text{Bi}^{46+}$, reaching electron temperatures up to \~{}120 eV. The maximum ionization balance achieved was$\text{Bi}^{3+}$, indicating that even higher intensities would be required to further enhance BEUV output. Post-processing of the hydrodynamic simulations revealed a spatial “sweet spot” for efficient EUV/BEUV emission located just beyond the critical surface. At$6.0 \times 10^{11} ~\mathrm{W} / \text{cm}^{2}$, its extent was$\sim 500 \mu ~\mathrm{m}$for$\text{Sn}, \sim 225 \mu ~\mathrm{m}$for Gd, and$\sim 125 \mu ~\mathrm{m}$for Bi. Conversion efficiency exhibited a nonlinear increase with laser intensity, rising more rapidly at higher intensities included in this work. Comparisons between LTE and optically thin NLTE spectral modeling showed that LTE overestimates emission while simplified NLTE underestimates it, underscoring the importance of accurate collisional-radiative modeling for reliable BEUV source optimization.
Language: English
Subfield: Nuclear and High Energy Physics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: hybrid
OpenAlex ID: W7204544111