EUV氟化钙物镜材料的各向异性去除机理研究

Due to its excellent optical properties, such as visible-transparent and high laser-induced damage threshold, the calcium fluoride single crystal (CaF2) has been as the irreplaceable material of object lens in ultraviolet lithography. With the development of 13.5 nm extreme ultraviolet lithography (EUV), it puts forward higher requirements on the surface accuracy and quality of CaF2 object lens. For example, the surface accuracy and RMS roughness must be in angstrom level and the substrate crystal structure approaches nondestructive state. However, due to the anisotropic mechanical properties and material removal of CaF2 crystal, the accurate establishment of anisotropic removal function as well as the achievement of consistent and controllable material removal during polishing process have become the keys factor to restrict the manufacture of super-precision aspheric surface of CaF2 lens. Therefore, this project will systemically investigate the behaviors, mechanism and control of the anisotropic material removal on CaF2 surface. Firstly, both the anisotropic mechanical properties and chemical etching rate will be studied. Secondly, the effect of crystal anisotropic behavior on the material removal and the surface damage will be explored. Then, the anisotropic removal model of CaF2 material will be established. As a result, through the experimental verification in magnetorheological finishing (MRF) polishing, the optimization processing on MRF polishing will be proposed to realize the super-smooth and nondestructive state of CaF2 aspheric surface.

氟化钙晶体基于其高透光率和高抗激光损伤阈值等优异的光学性能，已成为目前紫外光刻物镜系统中不可替代的透镜材料。随着紫外光刻进入曝光波长为13.5 nm的极紫外工艺，对氟化钙透镜的表面质量提出了更为苛刻的要求，如亚纳米级的面形精度和表面粗糙度，以及近零损伤的超精密表面。针对不同晶面和晶向上晶体力学性能和材料去除的差异，如何准确构建氟化钙材料的各向异性去除函数，实现其在抛光过程中的一致、可控去除，已成为制约高精度、超精密氟化钙非球面加工的关键。为此，本项目拟系统开展氟化钙晶体的各向异性去除行为、机理和控制研究。在阐明氟化钙材料的各向异性力学性能和化学刻蚀规律的基础上，深入揭示晶体各向异性对材料摩擦化学去除和表面损伤的影响机制，构建氟化钙材料各向异性去除模型，并结合磁流变抛光实验验证，最终研究提出适用于超光滑、低损伤氟化钙非球面加工工艺的优化方案。

氟化钙晶体基于其高透光率和高抗激光损伤阈值等优异的光学性能，已成为目前紫外光刻物镜系统中不可替代的透镜材料。随着紫外光刻进入曝光波长为13.5 nm的极紫外工艺，对氟化钙透镜的表面质量提出了更为苛刻的要求，如亚纳米级的面形精度和表面粗糙度，以及近零损伤的超精密表面。针对不同晶面和晶向上晶体力学性能和材料去除的差异，如何准确构建氟化钙材料的各向异性去除函数，实现其在抛光过程中的一致、可控去除，已成为制约高精度、超精密氟化钙非球面加工的关键。为此，本项目系统地开展了单晶氟化钙单点切削时材料的各向异性去除行为、机理研究，以及复杂工况环境下摩擦化学去除机理、影响因素研究。首先，研究了氟化钙晶体单点切削时的各向异性去除的行为与机理；其次，深入研究了氟化钙晶体摩擦化学去除行为及溶液pH值对摩擦化学的影响机制；最后，全面总结了前述研究结论与成果，提出氟化钙CMP优化措施，通过CMP方法获得了超光滑低损伤的氟化钙表面；并探究了氟化钙和单晶硅等各向异性材料的原子级材料去除极限。本项目研究成果符合国家高新科技发展的重大战略需求，不仅可以丰富纳米摩擦学和超精密加工的基础理论，而且也有助于推动我国CMP技术发展和光刻机设备国产化进程。