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 of 3D microstructure fabrication processes. In this work, an innovative 3D fabrication method for microlens arrays on curved surfaces is introduced. To fabricate the microlens array, a PMMA microlens array on a curved surface was used as the projection microlens array. A thick layer of positive tone photoresist SPR 220 was spin coated on a curved, titanium-coated aluminum substrate. A pre-designed 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, forming a 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 by using a Twyman-Green interferometer.