近日，荷兰代尔夫特理工大学的Yifeng Shao及其研究团队取得一项新进展。经过不懈努力，他们实现基于自动微分的波长复用多模EUV反射叠层成像术。相关研究成果已于2024年8月19日在国际知名学术期刊《光：科学与应用》上发表。

本研究介绍并测试了一种新型算法，该算法能够实现波长复用重建，从而提高测量吞吐量并引入数据多样性，以便对样本结构进行准确表征。为解决高次谐波产生（HHG）光源固有的不稳定性问题，本研究采用了一种模态方法，该方法通过一系列互不相干且独立的空间模态来表示照明的交叉密度函数。所提算法在主流机器学习平台上实现，该平台利用自动微分来管理模型复杂度的急剧增长，并使用GPU加速来加快计算速度。

通过优化超过2亿个参数，研究人员证明了该算法能够容纳实验不确定性，并在反射几何构型下实现接近衍射极限的分辨率。对带有20纳米高图案化金结构的硅基底晶片样本进行重建，凸显了该研究处理涉及众多参数的复杂物理相互关系的能力。这些结果确立了叠层成像术作为一种高效且准确的计量工具的地位。

据悉，叠层极紫外（EUV）衍射成像技术已成为半导体行业下一代计量解决方案的有希望候选者，因为它能够在纳米尺度上以反射几何构型对晶片样本进行成像。由于高次谐波产生（HHG）EUV光源取得了显著进步，以及计算用硬件和软件的发展，该技术最近受到了广泛关注。

附：英文原文

Title: Wavelength-multiplexed multi-mode EUV reflection ptychography based on automatic differentiation

Author: Shao, Yifeng, Weerdenburg, Sven, Seifert, Jacob, Urbach, H. Paul, Mosk, Allard P., Coene, Wim

Issue&Volume: 2024-08-19

Abstract: Ptychographic extreme ultraviolet (EUV) diffractive imaging has emerged as a promising candidate for the next generationmetrology solutions in the semiconductor industry, as it can image wafer samples in reflection geometry at the nanoscale. This technique has surged attention recently, owing to the significant progress in high-harmonic generation (HHG) EUV sources and advancements in both hardware and software for computation. In this study, a novel algorithm is introduced and tested, which enables wavelength-multiplexed reconstruction that enhances the measurement throughput and introduces data diversity, allowing the accurate characterisation of sample structures. To tackle the inherent instabilities of the HHG source, a modal approach was adopted, which represents the cross-density function of the illumination by a series of mutually incoherent and independent spatial modes. The proposed algorithm was implemented on a mainstream machine learning platform, which leverages automatic differentiation to manage the drastic growth in model complexity and expedites the computation using GPU acceleration. By optimising over 200 million parameters, we demonstrate the algorithm's capacity to accommodate experimental uncertainties and achieve a resolution approaching the diffraction limit in reflection geometry. The reconstruction of wafer samples with 20-nm high patterned gold structures on a silicon substrate highlights our ability to handle complex physical interrelations involving a multitude of parameters. These results establish ptychography as an efficient and accurate metrology tool.

DOI: 10.1038/s41377-024-01558-3

Source: https://www.nature.com/articles/s41377-024-01558-3