Characteristics of extreme ultraviolet emissions from interaction between delay-adjustable dual-wavelength laser and Sn target

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 an alternative drive laser for the next-generation LPP-EUV source. Compared with currently used CO<sub>2</sub> lasers, solid-state lasers have higher electrical-optical efficiency, more compact size, and better pulse shape tunability. Although limited to shorter operating wavelengths, the solid-state lasers have higher critical plasma density and optical depth. Consequently, 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 on an Sn slab target co-axially. The EUV energy and spectra of the Sn plasma are characterized at various delay times. It is demonstrated that compared with the 1.064-μm single pulse, the 0.532-μm pre-pulse laser with short delay times of 10 ns and 20 ns respectively results in a 4% increase in CE at 26° and 18% increase at 39°. The angular distribution of EUV energy is modulated by the 0.532-μm pre-pulse. An isotropic emission can be obtained within a certain delay time. The spectral feature near 13.5 nm is optimized, and a spectral purity of 12.2% is improved by 69%. The laser spot sizes of 0.3 mm and 1 mm for the pre-pulse are compared in the experiment. The results show that the 1-mm spot size has a better modulation effect on the EUV emission. 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 related to the plasma morphology, and defined as the ratio of the longitudinal size to 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 has guiding significance for optimizing the emission characteristics of solid-state laser driven EUV sources.