Microlens shape optimization through lithographic photoresist volume control

Microlenses and microlens arrays play an important role in a wide range of modern industrial applications, including imaging systems, fiber coupling, laser beam collimation, laser beam shaping, and many more. Lenses are commonly characterized by parameters such as diameter, radius of curvature (RoC), conical constant (K), and lens height (SAG). A common wafer-level fabrication method involves forming photoresist cylinders through photolithography, which are subsequently melted to create lens profiles in a process known as thermal reflow, and then transferred to a hard substrate via reactive ion etching. The size and volume of the photoresist structures directly influence the final lens shape after etching. Additionally, non-uniformity introduced during the fabrication process - especially after etching - can significantly affect the final lens shape and, consequently, device performance. To overcome these variations and enhance yield and uniformity, this paper presents a process to compensate for such variations using a lithography-based photoresist volume control strategy. This method introduces perforations directly into the photoresist cylinders prior to reflow using a direct writing laser (DWL) system. The photoresist volume can be adjusted from 0.66% to 40.71%. We also present an analysis of how variations in photoresist volume influence the resulting microlens shape. This approach provides a promising pathway for improving microlens uniformity in large-scale manufacturing.