Fabrication of long-focal-length plano–convex microlens array by combining the micro-milling and injection molding processes

Abstract

A uniform plano-convex spherical microlens array with a long focal length was fabricated by combining the micromilling and injection molding processes in this work. This paper presents a quantitative study of the injection molding process parameters on the uniformity of the height of the microlenses. The variation of the injection process parameters, i.e., barrel temperature, mold temperature, injection speed, and packing pressure, was found to have a significant effect on the uniformity of the height of the microlenses, especially the barrel temperature. The filling-to-packing switchover point is also critical to the uniformity of the height of the microlenses. The optimal uniformity was achieved when the polymer melts completely filled the mold cavity, or even a little excessively filled the cavity, during the filling stage. In addition, due to the filling resistance, the practical filling-to-packing switchover point can vary with the change of the filling processing conditions and lead to a non-negligible effect on the uniformity of the height of the microlenses. Furthermore, the effect of injection speed on the uniformity of the height of the microlenses was analyzed in detail. The results indicated that the effect of injection speed on the uniformity of the height of the microlenses is mainly attributed to the two functions of injection speed: transferring the filling-to-packing switchover point and affecting the distribution of residual flow stress in the polymer melt.

Ultralong focal length microlens array fabricated based on SU-8 photoresist

“…Based on the results of Fig. 7, the largest focal length is 4465 μm, which is much larger than that of a microlens fabricated by other methods [4][5][6][7]. The ultralong focal length is an advantage of this fabrication method.…”

“…The setup [7] was composed of a 633 nm wavelength laser light source, a lens system, a CCD camera, a guide, and a computer, as shown in Fig. 6.…”

“…Long focal length MLA is a vital optical element, which decides the performance of SHWS [2,3]. In recent years, many fabrication methods [4][5][6][7][8] were proposed to create a long focal length microlens or extend the focal length of MLA. The thermal reflow method has been proposed for many years [9,10].…”

Fabrication of Multiscale-Structure Wafer-Level Microlens Array Mold

“…For economic reasons, MLAs are usually manufactured by melting photoresist, ultraviolet (UV) imprinting, injection molding, two-photon polymerization, or other batch processes [14][15][16][17][18][19]. Melting of the photoresist layer provides a simple and practical method for MLA fabrication, but this conventional process is limited by the high relief depth of the MLA.…”

“…In addition, it is difficult to control the surface-shape error of microlenses [20]. In contrast, a high-precision MLA mold is used in the UV-imprinting process, which is followed by UV-curing the photoresist layer [16], and in the injection-molding process, in which soft optical resin material is injected into the mold to obtain MLAs with the desired optical properties. Since the width to thickness ratio of the wafer MLA is very large, the mold-based UV-imprint process is currently used in industrial wafer MLA manufacturing.…”