The fabrication of non-spherical microlens arrays utilizing a novel SU-8 stamping method

SU-8 photoresist (PR) is an excellent material for optical applications due to its high refractive index and high transmittance at visible and near-IR wavelengths. This paper reports a simple and novel method of fabricating non-spherical microlens arrays by soft stamping unexposed SU-8 PR. An SU-8 based stamp composed of micro-nozzle arrays and a reservoir structure are first fabricated on a glass substrate using a process of dosage control exposure. The unexposed SU-8 encapsulated in the crosslinked SU-8 shell is used as the 'ink' for the stamping process. The present SU-8 microlens array is then formed by stamping the formed SU-8 structure on a bare glass substrate at a temperature higher than the glass transition temperature (T(g)) of the unexposed SU-8 PR. A non-spherical lens with various radii of curvature structure can be formed by controlling the working temperature during the stamping process. Microlens arrays with diameters ranging from 50 to 500 mu m are successfully fabricated using this novel process, with the height of the fabricated microlens reaching 200 mu m. Experimental investigation results indicate that the formed microlenses have a good surface profile, good uniformity and good optical properties. The measured numerical apertures (NA) of the fabricated microlens range from 0.22 to 0.58. The relationships between the focal length of the fabricated microlens and the lens dimension as well as the process temperature are systematically investigated. The method presented in this study will substantially impact the process for microlens array fabrication.