Microtransfer molding of SU-8 micro-optics

Abstract

SU-8 is a very promising polymer 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 patterning of SU-8 is relatively common, molding of SU-8 is more difficult due to challenges with solvent removal and cross linking. In this paper, we discuss techniques for micromolding of micro- and nano-optics in SU-8. Elastomeric mold templates are first cast from master structures fabricated using standard techniques. The elastomeric templates are then used in low pressure molding processes to produce high-fidelity refractive and diffractive micro-optics in SU-8. The use of the elastomeric replica mold enables realization of a wider variety of optical surfaces than can be achieved with conventional lithographic patterning in SU-8, and further enables conformal fabrication of SU-8 micro-optics on non-planar surfaces. Molding processes and experimental results for both thin (diffractive) and thick (refractive) elements are presented. Replication of SU-8 micro-optics on both planar and non-planar surfaces, and hybrid processes combining molding and lithographic exposure are demonstrated.