Monolithically Integrated Optical Through-Silicon Waveguides for 3D Chip-to-Chip Photonic Interconnects

By Francesco Villasmunta ^1,2; Patrick Heise ¹; Manuela Breiter ⁴; Sigurd Schrader ¹; Harald Schenk ^5,6; Martin Regehly ¹  and Andreas Mai ^1,3
1 Technical University of Applied Sciences Wildau, Germany
² Brandenburg University of Technology Cottbus-Senftenberg, Germany
³ IHP - Leibniz-Institut für innovative Mikroelektronik, Germany
⁴ TU Ilmenau, Center of Micro- and Nanotechnologies, Germany 
⁵ Brandenburg University of Technology Cottbus-Senftenberg, Germany
⁶ Fraunhofer IPMS, Germany

Abstract:

The scaling limitations of electrical interconnects are driving the demand for efficient optical chip-to-chip links. We report the first monolithic integration of air-clad optical through-silicon waveguides in silicon, fabricated via Bosch and cryogenic deep reactive-ion etching. Rib, single-bridge, and double-bridge designs with 50 μm cores and up to 150 μm propagation lengths have been evaluated. Cryogenic-etched rib waveguides achieve the highest median transmission (66%, -1.80 dB), compared to Bosch-etched ribs (62%, -2.08 dB). Across all geometries, 3 dB alignment windows range from 9.3 μm to 49.2 μm, with Bosch-etched double-bridge waveguides providing the broadest tolerance. We show that geometric fidelity outweighs sidewall roughness for transmission and alignment in these large-core, multimode optical through-silicon waveguides. This technology provides a scalable, complementary metal-oxide semiconductor-compatible pathway toward 3D photonic interconnects.

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