To guide the optical field inside a TSV as optical-waveguide or fiber, its core has to have a higher refractive index than the surrounding material. Comparing different material / technology options it turned out that thermal grown silicon dioxide (SiO2) is a perfect candidate for the cladding (nSiO2 = 1.4525 at 850 nm). In combination with SiO2 as the adjacent polymer layer, the negative resist SU-8 is very well suited as waveguide material (nSU-8 = 1.56) for the core. Here, we present the fabrication of an optical polymer based multimode waveguide in TSVs proved on waferlevel using SU-8 as core and SiO2 as cladding. The process resulted in a defect-free filling of waveguide TSVs with SU-8 core and SiO2 cladding up to aspect ratio (AR) 20:1 and losses less than 3 dB.
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S. Killge, S. Charania, K. Richter, N. Neumann, Z. Al-Husseini, D. Plettemeier, J. W. Bartha, "Realization of optical multimode TSV waveguides for Si-Interposer in 3D-chip-stacks ," Proc. SPIE 10232, Micro-structured and Specialty Optical Fibres V, 102320T (16 May 2017);