Molding and hot forming techniques for fabricating plastic aspheric lenses with high blue-light transmittance
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The purpose of this paper is to use micro-electroforming and hot-embossing technology as an alternative to high-cost precision cutting or traditional injection molding in the fabrication of plastic aspheric lenses with high Blu-Ray transmittance. The female dies for the aspheric lenses are fabricated from UV-cured SU-8 polymer via electrostatic attraction. However, SU-8 has a 405 nm Blu-Ray transmittance of only roughly 40–50%, which is not appropriate for use in high-density optical pickup systems. This paper, therefore, uses low stress, low surface roughness, nickel micro-electroforming and molding technologies and employs a micro hot-embossing system to form aspheric lenses with high Blu-Ray transmittance from COC plastic (transmittance: 88% at 405 nm). The resulting lenses have a clear aperture of approximately 1 mm and a numerical aperture of roughly 0.6. The electroforming mold has a roughness of approximately 8 nm as measured by AFM. The roughness of COC (n = 1.53) aspheric lenses after hot-embossing is approximately 146 nm (300 µm × 220 µm) as measured by white light interferometer (WYKO). The shape precision of the hot-embossing COC and original SU-8 (n = 1.67) aspheric lenses can be controlled with approximately 2.638% error. The spot size of the hot-embossed COC and original SU-8 aspheric lenses can be controlled with approximately 11% error. This error should account for the material refractive index difference and the shape error. The roughness and spot size were also tested using different pressing temperatures and forces. This technology could be developed to fabricate lenses in Blu-Ray 405 nm micro-optical systems.