CC BY 4.0Villasmunta, FrancescoFrancescoVillasmuntaHeise, PatrickPatrickHeiseBreiter, ManuelaManuelaBreiterSchrader, Sigurd K.Sigurd K.SchraderSchenk, HaraldHaraldSchenkRegehly, MartinMartinRegehlyMai, AndreasAndreasMai2025-10-132025-10-132026https://publica.fraunhofer.de/handle/publica/497266https://doi.org/10.24406/publica-572310.1109/JSTQE.2025.361500110.24406/publica-57232-s2.0-105017445768The 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.entrueOptical interconnectionsSilicon photonicsThree-dimensional integrated circuitsThrough-silicon vias (TSV)journal article