Robust 3D surface recovery by applying a focus criterion in white light scanning interference microscopy

Area of Science:

• Optical Metrology
• Surface Characterization
• Microscopy

Background:

• White light scanning interference (WLSI) microscopy offers precise surface topography but struggles with low-reflectivity or rough surfaces, such as corroded materials.
• Shape from focus (SFF) is an alternative technique that uses surface texture and focus metrics for 3D reconstruction.

Purpose of the Study:

• To propose and evaluate a novel hybrid technique, SFF-WLSI, combining SFF and WLSI for enhanced 3D surface metrology.
• To assess the performance of SFF-WLSI across varying surface roughness and reflectivity conditions.

Main Methods:

• Developed a new technique, SFF-WLSI, integrating the Tenegrad Variance (TENV) focus metric into WLSI image acquisition.
• Conducted extensive simulations to test SFF-WLSI under diverse surface conditions.
• Validated simulation findings using a Mirau-type microscope on a lapping specimen and a corroded metallic sphere.

Main Results:

• Simulations demonstrated that SFF-WLSI provides accurate measurements for surfaces with varying roughness and reflectivity.
• The proposed SFF-WLSI technique outperformed conventional WLSI and standalone SFF methods in simulations.
• Experimental validation showed high accuracy on a flat specimen (Ra=0.055 μm, σ=0.008 μm) and superior 3D reconstruction of a corroded sphere compared to WLSI.

Conclusions:

• SFF-WLSI significantly enhances the accuracy and reliability of 3D surface topography measurements, especially for challenging surfaces.
• The TENV focus metric applied within the SFF-WLSI framework effectively overcomes limitations of traditional WLSI for rough and low-reflectivity samples.