Fast replication of out-of-plane microlens with polydimethylsiloxane and curable polymer (NOA73)

Abstract

Out-of-plane microlens, as its in-plane counterpart, is an important micro optics component that can be used in building integrated micro-optic systems for many applications. In earlier publications from our group, an ultra violet (UV) lithography based technique for out-of-plane microlens fabrication was reported. In this paper, we report a replication technology for time-efficient fabrication of out-of-plane microlens made of a curable polymer, NOA73. Microlens of cured SU-8 polymer was fabricated using a unique tilted UV lithography process, polydimethylsiloxane (PDMS) was molded using the resulting SU-8 master to form a negative mold, curable polymer NOA73 was then casted in the PDMS mold and out-of-plane microlens replica made of NOA73 was finally obtained after curing. The entire replication process took less than 5 h. Since PDMS negative mold was reusable, multiple replications of the microlens could be done with the same mold and each replication only took about 30 min. Scanning electron microscopic (SEM) images showed that NOA73 microlens replica had almost identical shape as the SU-8 master. In Comparison to the SU-8 microlens, microlens replica of UV curable polymer had slightly longer focal length and smaller numerical aperture due to the lower refractive index of NOA73. In addition, NOA73 microlens replica also had improved spectral transmission. Because of its compatibility with soft lithography technique, the reported replication process may also be used to integrate out-of-plane microlens into micro-opto-electro-mechanical-systems (MOEMS) and BioMEMS chips.