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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驱动激光在26°和39°上CE的提升分别达到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 an alternative drive laser for the next-generation LPP-EUV source. Compared with currently used CO2 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.
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Keywords:
- extreme ultraviolet sources /
- laser-produced plasma /
- pre-pulse /
- conversion efficiency /
- spectral purity
[1] Fomenkov I, Brandt D, Ershov A, Schafgans A, Tao Y, Vaschenko G, Rokitski S, Kats M, Vargas M, Purvis M, Rafac R, Fontaine B L, Dea S D, LaForge A, Stewart J, Chang S, Graham M, Riggs D, Taylor T, Abraham M, Brown D 2017 Adv. Opt. Technol. 6 173
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Lin N, Yang W H, Chen Y Y, Wei X, Wang C, Zhao J L, Peng Y J, Leng Y X 2022 Laser & Optoelectronics Progress 59 0922002
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图 1 实验布局示意图, 预脉冲 (绿色) 和主脉冲 (红色) 经f400透镜聚焦同轴入射Sn靶面, 入射角为0°, 两台EUV能量计与激光入射角度的夹角分别为26°和39°, EUV光谱仪与激光入射方向夹角为50°, 等离子体在可见光波段的发射通过f200透镜成像并由ICCD采集
Fig. 1. Schematic of the experimental layout. The pre-pulse (depicted in green) and the main pulse (depicted in red) are focused by a f400 lens co-axially and are incident onto the Sn target surface, the incident angle is 0°, the angle between the laser incident direction and the two EUV monitors are 26° and 39°, respectively, the observation angle of the EUV spectrometer is 50°, the visible band emission of the Sn plasma is imaged by a f200 lens and captured by an ICCD.
图 2 不同预脉冲光斑大小下的CE随延时变化趋势, 虚线为主脉冲单独作用时的CE值 (a) 0.3 mm光斑预脉冲下26° (红色方块)和39° (蓝色圆形)的CE随延时变化趋势; (b) 1 mm光斑预脉冲下26° (红色方块)和39° (蓝色圆形)的CE随延时变化趋势
Fig. 2. The dependency of CE on delay time for different pre-pulse spot sizes. The dashed lines mark the CE values when the main pulse irradiates the target without the pre-pulse: (a) The dependency of 26° (red square) and 39° (blue circle) CE on the delay time for 0.3 mm pre-pulse spot; (b) the dependency of 26° (red square) and 39° (blue circle) CE on the delay time for 1 mm pre-pulse spot.
图 3 不同预脉冲光斑大小下的全谱归一化EUV光谱, 图中紫色阴影区域对应EUV带内辐射波段 (a) 0.3 mm光斑预脉冲作用时不同延时下的EUV光谱; (b) 1 mm光斑预脉冲作用时不同延时下的EUV光谱
Fig. 3. The normalized EUV spectra at different pre-pulse spot sizes, the violet shadow area corresponds to 13.5 nm 2% bandwidth: (a) The EUV spectra at different delay times for 0.3 mm pre-pulse spot; (b) the EUV spectra at different delay times for 1 mm pre-pulse spot.
图 4 0.3 mm (黑色方块) 与1 mm (红色圆形) 预脉冲光斑大小下SP随延时变化趋势
Fig. 4. Dependency of SP on delay for 0.3 mm (black square) and 1 mm (red circle) laser spot sizes.
图 5 等离子体成像测量 (a)—(c) 0, 50, 1000 ns时的等离子体图像, 图像左侧为靶面位置, 激光入射方向为从右到左, 所有图像设置为相同对比度范围; (d) 0.3 mm (黑色方块) 和1 mm (红色圆形) 预脉冲光斑大小下的等离子体纵向尺寸随延时变化趋势; (e) 0.3 mm (黑色方块) 和1 mm (红色圆形) 预脉冲光斑大小下等离子体纵向中心位置随延时变化趋势
Fig. 5. The plasma imaging measurements: (a)–(c) The plasma images at 0, 50, 1000 ns, the left side of the image is the target surface, and the laser is incident from the right side, all images are set to the same contrast ratio; (d) the dependency of longitudinal size of the plasma on delay time for 0.3 mm (black square) and 1 mm (red circle) laser spot sizes; (e) the dependency of longitudinal central position of the plasma on delay time for 0.3 mm (black square) and 1 mm (red circle) laser spot sizes.
图 6 CE比例和等离子体纵向/横向长度比 (a) 0.3 mm光斑预脉冲下39°和26°的CE之比和等离子体纵/横比随延时变化趋势; (b) 1 mm光斑预脉冲下39°和26°的CE之比和等离子体纵/横比随延时变化趋势
Fig. 6. The CE ratio and the longitudinal/lateral size ratio of the plasma: (a) The dependency of the 39°/26° CE ratio and the longitudinal/lateral size ratio on delay time for 0.3 mm pre-pulse; (b) the dependency of the 39°/26° CE ratio and the longitudinal/lateral size ratio on delay time for 1 mm pre-pulse.
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[1] Fomenkov I, Brandt D, Ershov A, Schafgans A, Tao Y, Vaschenko G, Rokitski S, Kats M, Vargas M, Purvis M, Rafac R, Fontaine B L, Dea S D, LaForge A, Stewart J, Chang S, Graham M, Riggs D, Taylor T, Abraham M, Brown D 2017 Adv. Opt. Technol. 6 173
Google Scholar
[2] Lin N, Chen Y Y, Wei X, Yang W H, Leng Y L 2023 High Power Laser Sci. Eng. 11 e64
Google Scholar
[3] Chen Y Y, Liu Z X, Lin N 2025 Opt. Lasers Eng. 189 108946
Google Scholar
[4] 林楠, 杨文河, 陈韫懿, 魏鑫, 王成, 赵娇玲, 彭宇杰, 冷雨欣 2022 激光与光电子学进展 59 0922002
Lin N, Yang W H, Chen Y Y, Wei X, Wang C, Zhao J L, Peng Y J, Leng Y X 2022 Laser & Optoelectronics Progress 59 0922002
[5] Nowak T S, Yokotsuka T, Fujitaka K, Moriya M, Ohta T, Kurosu A, Sumitani A, Fujimoto J 2010 EUVL Workshop p2
[6] Versolato O O, Sheil J, Witte S, Ubachs W, Hoekstra R 2022 J. Opt. 24 054014
Google Scholar
[7] Sistrunk E, Alessi D, Bayramian A, Chesnut K, Erlandson A, Galvin T, Gibson D, Nguyen H, Reagan B, Schaffers K, Siders C, Spinka T, Haefner C 2019 Proc. SPIE 11034 1103407
[8] Campos D, Harilal S S, Hassanein A 2010 Appl. Phys. Lett. 96 151501
Google Scholar
[9] Harilal S S, Sizyuk T, Hassanein A, Campos D, Hough P, Sizyuk V 2011 J. Appl. Phys. 109 063306
Google Scholar
[10] Fujioka S, Nishimura H, Nishihara K, Sasaki A, Sunahara A, Okuno T, Ueda N, Ando T, Tao Y, Shimada Y, Hashimoto K, Yamaura M, Shigemori K, Nakai M, Nagai K, Norimatsu T, Nishikawa T, Miyanaga N, Izawa Y, Mima K 2005 Phys. Rev. Lett. 95 235004
Google Scholar
[11] Freeman J R, Harilal S S, Verhoff B, Hassanein A, Rice B 2012 Plasma Sources Sci. Technol. 21 055003
Google Scholar
[12] Ando T, Fujioka S, Nishimura H, Ueda N, Yasuda Y, Nagai K, Norimatsu T, Murakami M, Nishihara K, Miyanaga N, Izawa Y, Mima K, Sunahara A 2006 Appl. Phys. Lett. 89 151501
Google Scholar
[13] Harilal S S, O'Shay B, Tillack M S, Tao Y, Paguio R, Nikroo A, Back C A 2006 J. Phys. D: Appl. Phys. 39 484
Google Scholar
[14] Hayden P, Cummings A, Murphy N, O’Sullivan G, Sheridan P, White J, Dunne P 2006 J. Appl. Phys. 99 093302
Google Scholar
[15] Lan H, Wang X B, Zuo D L 2016 Chin. Phys. B 25 035202
Google Scholar
[16] Si M Q, Wen Z L, Zhang Q J, Dou Y P, Li B C, Song X W, Xie Z, Lin J Q 2023 Acta Physica Sinica 72 065201
Google Scholar
[17] Freeman J R, Harilal S S, Hassanein A 2011 J. Appl. Phys. 110 083303
Google Scholar
[18] Freeman J R, Harilal S S, Hassanein A, Rice B 2013 Appl. Phys. A 110 853
Google Scholar
[19] Cummins T, O'Gorman C, Dunne P, Sokell E, O'Sullivan G, Hayden P 2014 Appl. Phys. Lett. 105 044101
Google Scholar
[20] Tao Y, Tillack M S, Harilal S S, Sequoia K L, Najmabadi F 2007 J. Appl. Phys. 101 023305
Google Scholar
[21] Tao Y, Tillack M S, Harilal S S, Sequoia K L, Burdt R A, Najmabadi F 2007 Opt. Lett. 32 1338
Google Scholar
[22] Garbanlabaune C, Fabre E, Max C E, Fabbro R, Amiranoff F, Virmont J, Weinfeld M, Michard A 1982 Phys. Rev. Lett. 48 1018
Google Scholar
[23] Wang T Z, Hu Z L, He L, Lin N, Leng Y X, Chen W B 2025 Vacuum 231 113805
Google Scholar
[24] 胡桢麟, 何梁, 王天泽, 林楠, 冷雨欣 2025 中国激光 52 0601001
Google Scholar
Hu Z L, He L, Wang T Z, Lin N, Leng Y X 2025 Chin. J. Lasers 52 0601001
Google Scholar
[25] 何梁, 胡桢麟, 王天泽, 林楠, 冷雨欣 2025 激光与光电子学进展 62 0314001
Google Scholar
He L, Hu Z L, Wang T Z, Lin N, Leng Y X 2025 Laser Optoelectron. Prog. 62 0314001
Google Scholar
[26] 蔡懿, 王文涛, 杨明, 刘建胜, 陆培祥, 李儒新, 徐至展 2008 物理学报 57 5100
Google Scholar
Cai Y, Wang W T, Yang M, Liu J S, Lu P X, Li R X, Xu Z Z 2008 Acta Phys. Sin. 57 5100
Google Scholar
[27] Versolato O O 2019 Plasma Sources Sci. Technol. 28 083001
Google Scholar
[28] Morris O, O’Reilly F, Dunne P, Hayden P 2008 Appl. Phys. Lett. 92 231503
Google Scholar
[29] Schupp R, Torretti F, Meijer R A, Bayraktar M, Sheil J, Scheers J, Kurilovich D, Bayerle A, Schafgans A A, Purvis M, Eikema K S E, Witte S, Ubachs W, Hoekstra R, Versolato O O 2019 Appl. Phys. Lett. 115 124101
Google Scholar
[30] Tao Y, Harilal S S, Tillack M S, Sequoia K L, O'Shay B, Najmabadi F 2006 Opt. Lett. 31 2492
Google Scholar
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