Fabrication of Az4562 Refractive Microlenses Array for Light Enhancement on Optical Microsystems

This paper presents the fabrication of an array of high aspect ratio photoresist based refractive microlenses (ML) using photolithography and thermal reflow. First, and in order to evaluate and predict the MLs optical properties and physical dimensions, finite element analysis was done. These simulations helped to design the super high resolution chrome on soda lime glass photomask as well as the parameters for the lithographic processes. Then, an array of high aspect ratio structures (length 4.9 mm, width 30 μm and 5 μm spacing between adjacent structures) with 5 μm thickness were fabricated. The thermal reflow technique (using a hotplate) was applied and an array of MLs measuring 5 and 32 μm at the vertex and radius, respectively, was achieved. When the photoresist (PR) is heated up above its glass transition temperature, it melts and the surface tension effect causes the fabricated microstructure to obtain the spherical lens profile. The hotplate thermal reflow is simple and easy to control, thus permitting the fabrication of smooth and homogeneous surfaces essential for good quality refractive microlenses.