Zhou Yi, Tang Yan, Yang Yong, Hu Song
State Key Laboratory of Optical Technologies for Microfabrication, Institute of Optics and Electronics, Chinese Academy of Sciences, Chengdu 610209, China.
University of Chinese Academy of Sciences, Beijing 100049, China.
Micromachines (Basel). 2017 Oct 26;8(11):319. doi: 10.3390/mi8110319.
Scanning broadband light interferometry (SBLI) has been widely utilized in surface metrology due to its non-contact and high-accuracy method. In SBLI, phase evaluation through Fourier Transform (FT) is a prevalent and efficient technique, where the topography measurement can often be achieved through one interferogram. Nevertheless, the accuracy of the FT method would be significantly influenced by intensity modulation depth: "the lower the modulation of the pixel, the higher the error probability of its phase assignment". If the structure has a large enough range along the -axis, several areas in an individual interferogram would be weakly modulated due to the limited depth of focus (DOF). In this paper, we propose an advanced FT-based method when it comes to large-height structures. Spatial modulation depth is first calculated for each interferogram independently. After that, a binary control mask is reasonably constructed to identify the pixels that are valid for phase unwrapping. Then, a phase stitching method along the -axis is carried out to conduct the large-height topography measurement within a giving field of view. The theoretical principle, simulation, and experimental validation are elaborated to demonstrate that the method can achieve an improved robustness for the reconstruction of large-range microstructures, the advantages of which include the elimination of stepping errors, the suppression of light fluctuations, and the freedom of a limited DOF.
扫描宽带光干涉测量法（SBLI）因其非接触式和高精度的测量方法，已在表面计量学中得到广泛应用。在SBLI中，通过傅里叶变换（FT）进行相位评估是一种普遍且有效的技术，通常通过一幅干涉图就能实现形貌测量。然而，FT方法的精度会受到强度调制深度的显著影响：“像素的调制越低，其相位赋值的误差概率就越高”。如果结构在z轴方向上有足够大的范围，由于景深（DOF）有限，单个干涉图中的几个区域会被弱调制。在本文中，针对大高度结构，我们提出了一种基于FT的先进方法。首先独立计算每个干涉图的空间调制深度。之后，合理构建一个二进制控制掩码，以识别对相位解缠有效的像素。然后，沿z轴进行相位拼接方法，以在给定视场内进行大高度形貌测量。详细阐述了理论原理、仿真和实验验证，以证明该方法能够提高大范围微结构重建的鲁棒性，其优点包括消除阶梯误差、抑制光波动以及不受有限景深的限制。
