Sohn Martin Y, Barnes Bryan M, Silver Richard M
Engineering Physics Division, National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, MD 20899, USA.
Optik (Stuttg). 2018 Mar;156:635-645. doi: 10.1016/j.ijleo.2017.11.206. Epub 2017 Dec 2.
Accurate optics-based dimensional measurements of features sized well-below the diffraction limit require a thorough understanding of the illumination within the optical column and of the three-dimensional scattered fields that contain the information required for quantitative metrology. Scatterfield microscopy can pair simulations with angle-resolved tool characterization to improve agreement between the experiment and calculated libraries, yielding sub-nanometer parametric uncertainties. Optimized angle-resolved illumination requires bi-telecentric optics in which a telecentric sample plane defined by a Köhler illumination configuration and a telecentric conjugate back focal plane (CBFP) of the objective lens; scanning an aperture or an aperture source at the CBFP allows control of the illumination beam angle at the sample plane with minimal distortion. A bi-telecentric illumination optics have been designed enabling angle-resolved illumination for both aperture and source scanning modes while yielding low distortion and chief ray parallelism. The optimized design features a maximum chief ray angle at the CBFP of 0.002° and maximum wavefront deviations of less than 0.06 λ for angle-resolved illumination beams at the sample plane, holding promise for high quality angle-resolved illumination for improved measurements of deep-subwavelength structures using deep-ultraviolet light.
对于尺寸远低于衍射极限的特征进行基于光学的精确尺寸测量，需要深入了解光学柱内的照明情况以及包含定量计量所需信息的三维散射场。散射场显微镜可以将模拟与角度分辨工具表征相结合，以提高实验与计算库之间的一致性，从而产生亚纳米级的参数不确定性。优化的角度分辨照明需要双远心光学系统，其中由柯勒照明配置定义的远心样品平面和物镜的远心共轭后焦平面（CBFP）；在CBFP处扫描孔径或孔径源，可以在最小失真的情况下控制样品平面处的照明光束角度。已经设计了一种双远心照明光学系统，可实现孔径和源扫描模式的角度分辨照明，同时产生低失真和主光线平行度。优化设计的特点是，在CBFP处主光线的最大角度为0.002°，在样品平面处角度分辨照明光束的最大波前偏差小于0.06λ，有望实现高质量的角度分辨照明，以改进使用深紫外光对深亚波长结构的测量。
