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Laser-produced plasma extreme ultraviolet (LPP-EUV) source is one of the key technologies in advanced lithography systems. Recently, solid-state lasers have been proposed as the alternative drive laser for the next-generation LPP-EUV source. Compared to currently used CO2 lasers, solid-state lasers are of higher electrical-optical efficiency, more compact size, and better pulse shape tunability. Whereas limited to shorter working wavelengths, the plasma critical density and optical depth for the solid-state lasers are higher. The consequent re-absorption and spectral broadening cause lower conversion efficiency (CE). Therefore, to optimize EUV emission features and improve CE, a 0.532 μm pre-pulse laser is utilized in this work to modulate the plasma density distribution. The pre-pulse and a 1.064 μm Nd: YAG laser (the main pulse) are incident to a Sn slab target co-axially. The EUV energy and spectra of the Sn plasma are characterized at various delay times. We demonstrate that the CE is increased by 4% (26°) and 18% (39°) at short delay times of 10 and 20 ns respectively compared to 1.064 μm single-pulse irradiation. The angular distribution of EUV energy is modulated by the 0.532 μm pre-pulse. An isotropic emission can be obtained at a certain delay time. The spectral feature near 13.5 nm is optimized and a spectral purity of 12.2% is obtained with an increase of 69%. The laser spot sizes of 0.3 mm and 1 mm for the pre-pulse are compared in the experiment. Results show that the 1 mm spot size has a better modulation effect on the EUV emissions. Moreover, the time-resolved visible-band plasma profile is captured by an ICCD with 1.6 ns gate width. The plasma size and the distance to the target surface are increased by the 0.532 μm pre-pulse, which suggests that the energy of the main pulse is deposited in the low-density pre-plasma plume instead of in the plasma near the target surface. The lower plasma density leads to an increase in CE and spectral purity. The angular distribution of EUV energy is found to be closely correlated to the plasma morphology, defined as the ratio of the longitudinal and lateral size of the plasma. This indicates that the variation of plasma morphology can influence the angular distribution of EUV energy, which is caused by the 0.532 μm pre-pulse. This work is instructive to the optimization of emission features in solid-state laser-driven EUV sources.
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Keywords:
- extreme ultraviolet sources /
- laser-produced plasmas /
- pre-pulse /
- conversion efficiency /
- spectral purity
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