Grating parameter characterization based on the combination of dark field geometry-guidance and model-assisted...

Accurate determination of both spatial period and height is essential for grating characterization using coherence scanning interferometry (CSI), particularly when the lateral period measurement limit of the CSI system and the grating height is smaller than the illumination wavelength. The accuracy of both grating period and height measurements are significantly constrained by the combined effects of limited lateral resolution and edge-induced interferometric signal superposition. In this paper, we propose a method that combines dark field geometry guidance with model-assisted interferometric processing to enable accurate grating characterization under resolution-limited conditions without modifying the existing hardware. Dark field imaging enhances edge localization rather than the intrinsic optical resolution of the system, thereby enabling subpixel grating-edge localization and geometric referencing. In simulation, the grating period estimation achieves an error below ±2%. Subsequently, the accurately localized grating edge geometry is incorporated into Fourier-domain analysis under the guidance of a Fourier-optics-based interferometric signal formation model, through which redundant superposed interferometric components induced by grating edges are effectively identified and eliminated, thereby enabling more reliable grating height calculation from the conditioned interferometric signal. Finally, the grating surface topography is reconstructed using a standard envelope-based CSI algorithm. As a representative validation, experiments on a Bruker step height standard demonstrate that the proposed combination of dark field geometry guidance and model-assisted interferometric processing achieves accurate step width estimation and effectively eliminates redundant interferometric components in the vicinity of step edges. In addition, measurements performed on the PTB RS-N resolution standard, which contains gratings with different spatial periods, further validate the robustness of the proposed method. Experimental results show that the proposed method improves both grating period determination and grating height measurement under resolution-limited conditions. For a grating with a 2 µm period, the proposed method reduces the signed relative height error of the reconstructed profile from 5.6% to 0.3% compared with conventional median filtering.