3D fabrication of spherical microlens arrays on concave and convex silica surfaces

Three-dimensional (3D) microlens arrays are a group of micro-optics structures on curved surfaces that offer unique optical functions such as wide field of view in a compact arrangement. This paper reports an improved rapid lapping and molding system for 3D fabrication of spherical microlens arrays on non-planar surfaces including concave and convex surfaces for optical applications. Unlike traditional approaches for 3D micro patterns on curved silicon/silica substrates, this research demonstrates a low-cost and efficient chemical mechanical polishing process by lapping precision microlenses with steel balls and diamond slurries. Different from lapping on plane surfaces, lapping parameters for each micro cavity need to be accurately calculated and controlled to obtain microlenses with same apertures. Therefore, a micro wear model for micro cavity lapping process was established to calculate cavity sag height with the knowledge of down force, relative velocity and lapping time. Several groups of microlenses lapping were then conducted under the same conditions to validate the micro wear model. Guided by the micro wear model, two groups of microlens arrays were fabricated on concave and convex surfaces respectively. The shape accuracy and surface texture of the microlens arrays were evaluated by using a white light interferometer. Afterwards, the silica molds were coated with graphene film and then utilized to copy the 3D microstructures onto polymers by rapid surface molding. The wide angle imaging characterization of the 3D polymeric microlens arrays was illustrated by a simple optical setup and the measured MTF curves using the slanted-edge method. The improved manufacturing platform demonstrates a new approach for 3D microlens arrays fabrication on hard substrates in a cost effective way.