Micromirror fabrication for co-packaged optics using 3D nanoimprint technology

Co-packaged optics (CPO) is a key technology for addressing power bottlenecks in datacenters by integrating optical and electrical components and replacing electrical wiring with optical links. In the CPO module where silicon chips are embedded on the substrate and polymer waveguides are integrated as optical connections, a pair of 3D micromirrors can achieve low-loss and wideband optical coupling from silicon photonics to polymer waveguides. The shape of the polymer micromirror patterned by grayscale photo lithography depends on process conditions and requires high fabrication accuracy. In this study, photonanoimprint technology is adopted for stable micromirror fabrication. The imprint process for a polymer micromirror was considered from a hyperelastic analysis using the finite element method. The master mold was prepared using grayscale lithography with photosensitive polyimide as a template of a polydimethylsiloxane (PDMS) replica mold. The micromirror fabrication was demonstrated on a 4-in. silicon wafer. By imprinting into a guide groove structure with a PDMS replica mold, over 30  μm-height micromirrors were stably obtained multiple times by a step-and-repeat imprint. The linear part of the patterned mirror by the imprint process was more than 23.5  μm for four times imprinting, and the fabricated mirror shape was improved compared with grayscale lithography. The total height misalignment is 5  μm for 12 mirrors in four imprints, and 70% coupling efficiency in calculation was achieved.