A novel boundary-confined method for high numerical aperture microlens array fabrication

Abstract

In this paper, we present a technique to improve microlens array (MLA) uniformity after the thermal reflow process. It achieved high uniformity and high-numerical aperture (NA) simultaneously without sacrificing the fill factor. In order to improve the fill factor, a residual photoresist (PR) between the photoresist cylinders is used to make photoresist flow outward in standard thermal reflow processes. The PR cylinders, however, merge together easily due to an inexact reflow time and temperature distribution. This results in low uniformity and small lens height or low NA. We propose a boundary-confined method to pattern thin PR holes to prevent PR microlenses from merging together even after a long reflow time. Thick PR cylinders are patterned inside thin PR holes serving as boundaries. PR microlenses are formed after reflowing the thick photoresist cylinders. Both the uniformity and the height of a microlens can be well controlled. Besides, the fill factor is high due to the high resolution at the thin photoresist layer in photolithography. Our results show that the microlens is approximately a hemispherical profile. The gap between microlenses with a 48 µm diameter in hexagonal arrangement is 2 µm and the height of the microlens is 22 µm.