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激光-等离子体极紫外(LPP-EUV)光源是先进光刻系统中的核心子系统之一.近年来，固体激光逐渐成为新一代LPP-EUV光源的候选驱动激光方案.然而，由于工作波长较短，固体激光具有较高的等离子体临界密度和光厚，导致激光-极紫外光能量转换效率(CE)较低.针对这一问题，本工作提出采用波长为0.532 μm的预脉冲激光对等离子体密度进行调制，对预脉冲和波长为1.064 μm的Nd:YAG驱动激光（主脉冲）在不同延时下与Sn靶作用的辐射特性进行了测量.实验结果证明，0.532 μm预脉冲对Nd:YAG驱动激光的CE提升在26°和39°上分别达到4%和18%；实现了极紫外光能量角分布的有效调节，形成各向同性发射；实现了光谱形状优化，在预脉冲作用下光谱纯度达到12.2%，相较于仅主脉冲提升69%.此外，实验中还通过对等离子体发光轮廓进行时间分辨成像，证明了极紫外光能量角分布与等离子体形态的相关性.这表明0.532 μm预脉冲能够改变等离子体形态，进而影响EUV能量角分布特性.以上研究结果对固体激光驱动极紫外光源的辐射特性优化具有指导性意义.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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