- The paper demonstrates a dual-beam irradiation scheme that increases EUV conversion efficiency from 2.6% to 3.6%, achieving a 40% boost at fixed total energy.
- The methodology uses synchronized 20 mJ pulses on planar Sn targets and is validated through EUV diagnostics and radiation-hydrodynamic simulations.
- The enhanced plasma heating geometry and prolonged emission duration provide practical advantages for meeting high-NA lithography and scalable semiconductor manufacturing.
Dual-Beam 2-μm Laser Irradiation for Enhanced EUV Conversion Efficiency
Background and Motivation
The scalability of extreme ultraviolet (EUV) sources is a critical bottleneck for advanced semiconductor lithography, particularly as device technology nodes push towards angstrom-scale regimes and numerical aperture (NA) requirements intensify. State-of-the-art EUV sources, predominantly based on CO2 lasers, suffer from low wall-plug efficiency (~3%) and exorbitant power consumption at multi-kilowatt outputs (2606.08045). The emergence of solid-state lasers operating at 2 μm offers a promising avenue due to superior in-band conversion efficiency (CE) and potential for higher energy efficiency. However, the per-pulse energy constraints of 2-μm lasers—imposed by thermal damage thresholds—necessitate novel irradiation schemes to achieve the requisite output power for high-volume manufacturing.
Experimental Design and Methodology
The study addresses the fundamental limitation of per-beam energy in 2-μm-driven EUV sources by introducing simultaneous dual-beam irradiation of planar Sn targets using a Ho:YAG laser system (2090 nm, 20 ns). The experimental apparatus leverages a simple 50:50 beam splitter to divide the total energy (40 mJ) into two synchronized 20 mJ pulses, each tightly focused (30 μm spots) to preserve optimal intensity (1.6×1011 W/cm2) for EUV emission. Diagnostics include an EUV energy meter (Mo/Si mirror, Zr filter, Si x-ray diode) for CE quantification at 13.5 nm (2% bandwidth), pinhole imaging of the EUV source size, and Faraday cup ion TOF spectra for plasma characterization.
Radiation-hydrodynamic simulations (Star-2D) corroborate the plasma parameters observed experimentally, particularly electron temperature (~30 eV) and density (1020 cm−3), optimizing the ionic charge-state distribution for peak EUV yield.
Main Findings
The dual-beam irradiation scheme yields a 40% increase in in-band EUV CE compared to single-beam irradiation, elevating CE from 2.6% to 3.6% at the same total incident energy—a figure representing the highest reported for planar targets with 2-μm drivers. Notably, the improvement is not attributable to expanded source area alone; pinhole images confirm that the EUV source size (60–70 μm) remains consistent across both configurations and satisfies the sub-100 μm threshold for high-NA lithography tool requirements.
Temporal analysis of EUV emission waveforms indicates that dual-beam irradiation prolongs the emission duration (FWHM: ~200 ns vs. ~150 ns for single-beam), allowing for sustained plasma temperature (~30 eV) over a longer interval. This mitigates rapid cooling induced by three-dimensional plasma expansion, thereby increasing the total radiated EUV energy. Ion energy spectra, measured via TOF, further validate the equivalence of plasma conditions between the two schemes, reinforcing the reproducibility and scalability of the dual-beam approach.
Theoretical and Practical Implications
The demonstrated enhancement stems from improved plasma heating geometry: dual-beam irradiation increases the laser-heated volume and the EUV-emitting area, counteracting the limitations imposed by confined energy deposition and expansion cooling in single-beam layouts. The approach is both passive and modular, requiring only cascaded beam splitters—compatible with high-repetition-rate laser architectures (100 kHz) and without complex active stabilization.
Importantly, dual-beam irradiation narrows the gap between planar targets and optimized mass-limited droplet targets, which previously exhibited higher CE (up to 4%) but are less compatible with realistic high-volume manufacturing geometries. The methodology is extensible, suggesting further CE gains through additional beams and application to mass-limited targets and alternative wavelengths (Blue-X, water-window sources).
On the theoretical front, the results elucidate the interplay of laser wavelength, critical density, and plasma optical depth, with the 2-μm driver reducing self-absorption and narrowing EUV emission band, thus favoring spectral purity. The findings have direct implications for the design of next-generation, high-efficiency EUV sources critical for high-NA and hyper-NA lithography.
Speculation on Future Developments
Further beam multiplexing, leveraging passive optics, could facilitate multi-kW-class EUV sources with minimal incremental complexity. Extension to droplet targets and alternative irradiation geometries may yield conversion efficiencies exceeding those achieved here. The approach is also amenable to adaptation in emerging application spaces, including water-window soft x-ray and Blue-X sources, supporting a broader spectrum of advanced photonics and materials science platforms.
Conclusion
This work robustly establishes simultaneous dual-beam 2-μm laser irradiation as a viable, scalable method to boost EUV conversion efficiency for laser-produced plasma sources. The 40% CE enhancement at fixed total energy and the preservation of practical source size affirm its compatibility with high-volume, high-NA lithography systems. The method’s simplicity, scalability, and theoretical underpinning position it as a significant advancement towards energy-efficient, multi-kW EUV sources for next-generation semiconductor manufacturing and allied photonics applications (2606.08045).