Liu Chao, Xu Shuang, Liu Yufei, Xiao Zainan
Appl Opt. 2021 Dec 10;60(35):10830-10837. doi: 10.1364/AO.443118.
The dark-field defect inspection system occupies 70% of the market in the field of unpatterned wafer inspection. But the detection limit is still restrained by the haze signals. Signal-to-noise ratio (SNR) enhancement could effectively decrease the detection limit by decreasing the influence of the haze signals on the defect signals. The existing method of optimizing the inspection conditions, including beam path and collection channel, can enhance the SNR, but the effect is restrained by the system structure. The empirically designed aperture has been attempted to be used by blocking the scattering signals in a certain azimuth angle range. However, the performance is restrained, as a signal with a large SNR exists in the blocked scattering signals. In this paper, we propose a novel (we believe) aperture design method in the light of scattering field analysis to reduce the influence of the haze signals caused by the wafer surface roughness on the particle signals. On the basis of the bidirectional reflectance distribution function, apertures are designed according to the ratio field of the particle signal to haze and verified by the scattering model developed based on the tools of the National Institute of Standards and Technology. Additionally, incident conditions are optimized according to their influence on the SNR. It is noteworthy that the aperture designed under specific conditions cannot be used for all particles. Three aperture combination schemes are proposed in this paper, which can ensure the scattering characteristics such as intensity and sensitivity to meet the system requirements while improving the contrast. Simulation results verify that the detection limit decreases from 48 to 22 nm by introducing a well-designed aperture, with the case of -polarized incident light when the threshold is 3 and the incident angle is 72°. Multiaperture schemes have better performance over others, especially the one-to-one scheme.
暗场缺陷检测系统在无图案晶圆检测领域占据70%的市场份额。但其检测极限仍受雾度信号的限制。信噪比（SNR）增强可通过降低雾度信号对缺陷信号的影响有效降低检测极限。现有的优化检测条件（包括光路和采集通道）的方法可提高信噪比，但效果受系统结构限制。曾尝试使用经验设计的孔径来阻挡特定方位角范围内的散射信号。然而，由于被阻挡的散射信号中存在高信噪比信号，其性能受到限制。本文根据散射场分析提出一种新颖的孔径设计方法，以减少晶圆表面粗糙度引起的雾度信号对颗粒信号的影响。基于双向反射分布函数，根据颗粒信号与雾度的比值场设计孔径，并通过基于美国国家标准与技术研究院工具开发的散射模型进行验证。此外，根据其对信噪比的影响优化入射条件。值得注意的是，在特定条件下设计的孔径不能用于所有颗粒。本文提出三种孔径组合方案，在提高对比度的同时，可确保强度和灵敏度等散射特性满足系统要求。仿真结果验证，当阈值为3且入射角为72°时，在 - 偏振入射光的情况下，通过引入精心设计的孔径，检测极限从48纳米降至22纳米。多孔径方案比其他方案具有更好的性能，尤其是一对一方案。
