A new fabrication method of micro-lens arrays with element lens diameters of 20-50 μm was developed in the past research. In the method, epoxy micro-lens arrays are replicated using concave resist patterns as molds. In this paper, resist mold pattern profiles were investigated again in detail, and model curves to be fitted to pattern cross sections were looked over again. As a result, it was found that cross section curves of resist mold patterns were very accurately modeled by decomposing each curve into three parts of a central circular concave arc, a peripheral circular convex arc, and a tangential line connecting them. For this reason, using the new fitting curves, how parallel light rays incident in an element lens were refracted at the lens surface, and concentrated in a spot were investigated. It was clarified by the ray trace that parallel light rays were efficiently concentrated in a considerably small light spots by grace of the tangential line parts. Next, the ray trace results were compared with the experimental results. A micro-lens array fabricated under the same conditions was illuminated by a parallel light ray flux. As a result, the position where the traced rays were concentrated almost coincided with the position obtained experimentally. The diameters of concentrated light spots estimated by the ray trace also almost coincided with the actually observed ones. Thus, optical characteristics of microlens arrays fabricated by the new method were adequately qualified. The new method will be useful.
Lithography has been generally used for printing two-dimensional patterns on flat wafers. Recently, however, it is
also applied to a three-dimensional patterning for fabricating various MEMS (Micro Electro Mechanical Systems)
components. The purpose of this research is to develop a new method for fabricating micro-lens arrays. At first, resist
(Tokyo Ohka Kogyo, PMER LA-900PM) mold patterns with densely arrayed square or hexagonal concaves were
replicated by intentionally shifting the focal position of projection exposure. The size of resist-mold was 2 mm square,
and the initial thickness of the resist was 10 μm. Next, the wafer with the concave resist patterns was cut into small chips,
and each wafer chip was fixed at the bottom of a paper cup using an adhesive tape. Then the epoxy resin (Nissin resin,
Crystal resin Neo) was poured on the concave resist-mold patterns, and the resin was coagulated. Afterward, the
hardened resin was grooved along the wafer chip using a cutter knife, and the wafer chip with the resist-mold patterns
was forcibly removed using a pair of tweezers. Finally, both sides of the resin block were polished, and the thickness was
reduced. Although the transparency and roughness of the resin block surfaces should be improved, epoxy micro-lens
arrays were certainly fabricated. The mean values of curvature radius and lens height were 28.3μm and 4.9 μm,
respectively.
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