Topography Measurement of Large-Range Microstructures through Advanced Fourier-Transform Method and Phase Stitching...

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 z-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 z-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.