Metrology of steep slope surfaces in two-photon-lithography-manufactured micro-structures

Two-photon lithography (TPL) has emerged as a prominent additive manufacturing technique for fabricating customized micro-optical components. TPL offers exceptional design flexibility that enables the realization of complex, free-form geometries within built volumes of ∼1 mm in each dimension. However, as TPL can produce exotic surface topographies, it presents significant challenges for metrological characterization, particularly when evaluating steep surface features. Exact knowledge of the printed surface geometry enables quality assurance and the iterative correction of reproducible manufacturing inaccuracies. Conventional metrology tools, such as confocal microscopes and white light interferometers, exhibit limitations in measuring high-inclination surfaces due to constraints imposed by instrument parameters, as well as surface properties such as reflectivity and roughness. We investigate the maximum measurable surface angles. In addition, we explore the enhancement of surface detectability through the application of a thin gold coating via physical vapor deposition. Furthermore, we utilize metrology data to predict optical performance, demonstrating that the gold coating measurement accuracy leads to more reliable simulation results. We also confirm that the gold coating can be effectively removed by wet etching without adversely affecting optical performance. These findings offer valuable insights for quality control, product characterization, and process optimization in TPL-based micro-optical manufacturing.