Application of 3D glycerol-compensated inclined-exposure technology to an integrated optical pick-up head

This paper proposes a novel method for integrating inclined mirrors with precise angles and smooth surfaces in on-chip Fresnel lens optical pick-up-head applications. A glycerol-compensated inclined-exposure technology was adopted for 45° mirror pair fabrication. This technology can fabricate 19–90° inclined structures onto SU-8 negative tone resist with thickness from 100 to 1000 µm. Glycerol is employed as an index matching material during exposure to compensate for the refractive index difference between the air and SU8, thus extending the possible inclined angles from 54° (in air) down to 19° (in glycerol). To eliminate reflected induced patterns, CK-6020L resist is employed as an antireflection layer. The surface roughness, measured by AFM, is below 7 nm for various inclined angles. Device fabrication has been successfully completed and test results demonstrated a functional VCSEL (vertical cavity surface emitting laser) beam focusing on a 5 µm spot. Further beam size reduction into the sub-micrometer range requires an objective lens with high numerical aperture, which is not within the scope of this paper. The fabricated optical components are robust and accurately aligned. This new integrated approach provides an easy and low-cost fabrication method for optical pick-up heads eliminating the need for manual assembly.