A microlens array on curved substrates by 3D micro projection and reflow process

Microlens arrays are becoming increasingly important because of their widespread applications in optical, electronic, and energy fields. Currently, microlens array fabrication processes are mainly developed on planar substrates. For nonplanar substrates, existing fabrication methods suffer from various disadvantages. This is largely due to the inherent technical complexity associated with 3D microstructure fabrication processes. In this study, an innovative 3D fabrication method for microlens arrays on curved surfaces is introduced. To fabricate the microlens array, a polymethylmethacrylate microlens array on a curved surface was used as projection optics. A thick layer of positive photoresist SPR 220 was spin coated on a curved, titanium-coated aluminum substrate. A pre-selected pattern was projected onto the photoresist by using a home built exposure system. The development process resulted in micro cylinders on the curved substrate. A thermal reflow process was then performed on the cylinder array to form the microlens array. Experiments were conducted to evaluate the factors that affect the shapes of the microlenses. These factors include film thickness variation, exposure and development variation, slope of the substrate, height to width ratio and heating time in thermal reflow process. Finally microlenses were tested using a Twyman–Green interferometer. Results showed that the required shape of the microlenses achieved by controlling the process parameters.