A multi-layer vertically stacked all sol-gel fabrication technique on bulk silicon is proposed to realize vertical coupling from planar waveguides to 3-dimensional structures. The all sol-gel fabrication technique realizes the capability to stack ZnO sol-gel core layer and SiO2 sol-gel cladding and core separation layers. A major issue with sol-gel based fabrication is the appearance of cracks on the sol-gel layer, and these were improved by a slow cooling process of 1°C/min after 500°C

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... ure annealing in addition to a SiO2 surface cleaning using photoresist remover. As a result, a multi-layer vertically stacked structure with a ZnO core layer thickness of 300 nm and SiO2 cladding/core separation layer thickness of 1.5 µm was successfully fabricated on bulk Si.