Slope-deviation measurement of Fresnel-shaped mold surfaces

Molds are used to dictate their shape to other materials in embossing or filling processes. In optics fabrication especially, the exact surface slope of the polymer replica is of high relevance. The quality control of molds is challenging: non-invasive, optical metrologies struggle with shiny surfaces that minimize the scattering of light. In addition, the inspection of complex shaped molds with a stepped optical surface can be difficult. In response, the authors show a backward ray-tracing approach combined with fringe-reflection technique to determine the slopes of a Fresnel-shaped mold surface with topography features in the magnitude order of a quarter millimeter. The error is kept small by stitching together several measurements with different sample rotations.