Asymmetric focusing microlens array fabricated using off-axis lithography

Abstract

An asymmetric microlens with a given inclination angle was fabricated. Two circular pattern masks with different diameters were used to form a metal pattern and photoresist column on the substrate using the photolithography process. The metal pattern on the substrate was used to control the asymmetric microlens profile using thermal reflow. A lift-off process was applied to the first lithography to precisely define the metal pattern. A second lithography used deviation counterpoint exposure to pattern the photoresist column. The photoresist column was converted into a rubbery state when its temperature was increased to its glass transition temperature (Tg) during the thermal reflow. The asymmetric microlens structure was formed by shifting the arc vertex of the microlens toward one direction taking into account the fact that the copper coating surface has superior hydrophobicity to the silicon substrate surface. A 55° asymmetric microlens array was fabricated in this research by properly controlling the copper pattern size and the offset of two centers.