Fabrication of a transparent and self-assembled microlens array using hydrophilic effect and electric field pulling

Abstract

In this paper, we present a variable and self-assembled spherical microlens array (MLA) fabricated by the use of the hydrophilic effect under ultraviolet (UV)/ozone treatment. The optical power and surface roughness of the MLA were further enhanced by applying an external electric field. This method provides a fast, simple and low-cost process because it does not require lithography, heating, or etch-transfer processes. The MLA was made up of negative photoresist SU-8 (n = 1.63) on a glass substrate. Microlenses from 50 to 200 µm diameters with 1–4 min UV/ozone treatment time were successfully fabricated. The optical focusing power of the 100 µm diameter MLA was also improved by using the electric field of 1.7 and 3.4 V µm−1. A 10 µm thick shadow mask was used to define the UV/ozone treatment area to create more hydrophilic surfaces on an SU-8 photoresist base layer on a glass substrate. After hydrophilic zones were created, the glass substrate was immersed into a diluted SU-8 photoresist solution and then removed. Finally, the MLA was formed after applying a parallel electric field followed by an UV curing process. The MLA was kept in spherical shape, while the radius of curvature was changed. Focal lengths from 0.09 to 2.91 mm for a spherical MLA were experimentally demonstrated and investigated.