Refractive lenses in silicon micromachining by reflow of amorphous fluorocarbon polymer

Abstract

A new fabrication process for refractive microlenses making use of reflow of an amorphous fluorocarbon polymer, Cytop™, is described. This process is suitable for MEMS structures that require the integration of high-quality lenses producing MOEMS. The reflowed polymer has excellent optical properties and can be used throughout the visible and near-infrared wavelength range. The reflow step also results in curing of the polymer into an extremely stable and chemically resistant material. This enables MOEMS fabrication by a reverse-order protocol, i.e. the structuring of the reflow lens on the wafer precedes the silicon micromachining. In the first study, lenses with a diameter of 150 µm were fabricated. An optical analysis based on the sampled height profile revealed that the lenses worked as virtually perfect lenses, with a nominal focal length equal to ∼1550 µm, out to more than 50% of their diameter, while in the periphery the lens action was stronger, which leads to slightly pronounced side lobes in the shape of the focal spot. In the second parametric study, lenses with nominal diameters in the range of 25–150 µm were fabricated. In this study, the thickness of the Cytop layer was larger which resulted in lenses with a smaller radius of curvature of the surface and hence an increased focusing power. In particular, the lenses with 25–50 µm nominal diameters were found to have actual diameters somewhat larger than their nominal and a perfect lens shape all over their surface, yielding high-quality focusing lenses with f-numbers (focal length divided by useful diameter) as small as ∼1.9.

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