Characterization of hybrid molding and lithography for SU-8 micro-optical components

SU-8 is a very promising material for micro-optics. It is mechanically robust with high thermal and chemical resistance, has high transmission at visible and near-infrared wavelengths, and has relatively high refractive index after curing. While lithographic processing of SU-8 is relatively common, molding of SU-8 requires different processing parameters due to challenges with solvent removal and cross-linking. Understanding the effects of the molding process on SU-8 is necessary to optimize performance of molded micro-optical components and also to enable fabrication of more complex micro-optics through subsequent lithographic processing of molded structures. We examine properties of SU-8 as it undergoes the molding process. General characterization of SU-8 shrinkage and expansion is presented, and replication of sub-100-nm structures in SU-8 is demonstrated. Solvent content and refractive index as functions of processing parameters are also examined, along with analysis of SU-8's lithographic properties after undergoing the molding process. These characterizations further enable hybrid combinations of micromolding and lithographic processing to fabricate complex 3-D micro-optics and structures that are difficult or impossible to realize using conventional techniques.