Study on the measurement capability of point autofocus microscopy for steep surfaces beyond objective numerical aperture limits

Abstract As products continue to develop towards precision and intelligence, components are increasingly trending towards miniaturization. In the era of artificial intelligence, the market demand for small components is steadily growing. The point autofocus microscopy (PAM), with its small measurement spot and high vertical resolution, offers distinct advantages in the geometric measurement of small components. However, the maximum measurable angle of PAM is limited by the numerical aperture of the objective lens, typically not exceeding 20°, which makes it challenging to meet the measurement requirements for surfaces with high curvature. To achieve full-surface geometric measurement of microstructures, traditional methods rely on multiple measurements and data stitching, which compromises efficiency and may lead to cumulative errors. This paper proposes a method to enhance the maximum measurable angle of the PAM by utilizing diffuse reflected light based on the principle of point autofocus measurement. Additionally, by incorporating the principle of biased incidence, it provides the theoretical maximum measurable angles for three different tilt directions. Experiments were conducted using three optical diffusers with varying surface roughness levels as test specimens. The results indicate that the PAM can achieve sub-micrometer measurement accuracy for the relatively rough 220 and 600-grit optical diffusers specimens at tilt angles between 0° and 80°. In contrast, for the 1500-grit optical diffusers specimen, PAM fails to meet sub-micrometer accuracy once the tilt angle exceeds 10°.