二次电子像衬度模拟及在关键尺寸测量上的应用

The secondary electron (SE) images have the rich contrast, that the information as specimen topography and composition are concealed. SE simulation combined with specimen modeling provides a tool for translating this information. Especially for the environment SEM and complex nano-structure imaging, accurate simulation can achieve greater value of SE, but this model proposes more strict and complex requirements. This project is devoted to the improve SE theoretical model and develop application value of SE simulation in contrast analysis of the environmental SEM and CD accurate measurement. The research is mainly about three aspects: (1) Improving the model for SEs simulation will be implemented in this project. Traditional model considers the contribution from single electron excitation and bulk-plasmon decay for the SEs simulation, while ignoring the surface-plasmon excitation. It is proved by experimental results that the effect for surface-plasmon decay to SEs can be comparable to bulk-plasmon. As the major content of the improvement model, the effect of the surface-plasmon excitation on SE generation, transportation, cascade and emission can be modeled; (2) the simulation of environmental SE imaging based on the improved model is a new problem with the development of environmental SEM experiments. In this case, the collected SEs are more complicated by the influence of environmental materials, and the interpretation of the contrast mechanism requires the support of the simulation results. The simulation results provide with the theoretical support for the contrast explanation of environmental SE image; (3) we study the relationship between SE contrast and the geometric structure of the specimen in detail and develop the model based library for the CD metrology of the nanostructure.

二次电子（SE）像具有丰富的衬度，样品形貌、成分等信息隐含其中，SE模拟与样品建模相结合提供了翻译这些信息的工具。尤其环境SEM和复杂纳米结构成像，精确模拟能够实现SE像更大的价值，但这对模型的完善性提出更严格的要求。基于此，本课题从最根本的SE理论模型提高入手，发展其在环境SEM衬度分析和关键尺寸（CD）精确测量上的应用。研究主要为三个方面：（1）改进模型，传统模型主要考虑了单电子激发和体等离激元对SE的贡献，而忽略了表面等离激元激发，现已有实验证明表面激发对SE信号的贡献与体激发是可比的，对表面激发在SE产生、输运、出射过程的影响建模；（2）基于改进模型，模拟环境SE成像，这是随环境SEM发展而出现的新问题， SE信号受环境影响组成更复杂，模拟可以为SE像的衬度形成机制的解释提供理论支持并发展定量分析方法；（3）研究SE衬度与样品结构之间的关系，发展模型数据库方法用于CD测量。

本项目基于改进的二次电子(SE)产生、输运、出射的物理模型，模拟扫描电子显微镜(SEM)中出射的二次电子信号。基于对多种样品二次电子产额的模拟，建立形貌样品结构与SE衬度之间的对应关系，构建模型数据库，用于半导体纳米线宽关键尺寸(CD)的精确测量。研究将深度依赖的非弹性散射平均自由程引入蒙特卡罗方法中，用于研究电子与材料表面区域的相互作用。进一步考察了深度依赖的能量损失函数对SE产额的影响。基于改进的模型，进一步优化了二次电子信号曲线模型数据库(MBL)，这有力的推进了基于SE像的半导体纳米线宽测量相关国际标准的修订和完善。发布了相关领域ISO国际标准一项。研究从单一元素成分推广到化合物的纳米结构计算，从表面光滑结构扩展到波形、有衬底和涂层的复杂结构的SE产额计算，把MBL方法扩展到更加多的应用场景，有望推广到半导体纳米线宽、关键尺寸测量的实验室测量和工业在线测量。MBL方法技术原理清楚，数据库建立和误差控制技术成熟，具有很强的可操作性。研究建立了模型数据库思想的完整实现架构、建库与索引标准以及误差分析方法等。