Photonic Chiplet Integration on Optical Interposer: Can Hybrid Bonding Realize Passive Optical Coupling?

Silicon photonic integrated circuits (Si-PIC) have come a long way since Soref et al. first proposed the use of silicon waveguides in the mid-1980s. However, even after four decades, packaging and integration of silicon photonic systems remain an under-studied area. Most of the packaging activities have been focusing on Fibre-to-PIC coupling, and how to achieve the lowest possible coupling loss. In this paper, we look beyond Fibre-to-PIC coupling, and explore the possibility of PIC-to-PIC integration through a 2.5D optical interposer. We opine that hybrid bonding approach could be a suitable technology for passive PIC-to-PIC integration because it is based on A. gapless (oxide fusion bonding), B. bump-less electrical connection (direct Cu-to-Cu) and C. has a relatively high bonding accuracy (∼ 0.5 µm). Two optical coupling approaches were studied — grating-to-grating coupling, and evanescent coupling based on inverted taper pair. FDTD was used in this activity — first to understand the feasibility of the two coupling approaches and then how potential bonding misalignment can affect PIC-to-PIC coupling losses.