3D Heterogeneous Integration of Back-Emitting VCSEL Arrays via Flip-Chip Bonding for Co-Packaged Optics Systems

Flip-chip bonding is a promising technique for integrating Vertical-Cavity Surface-Emitting Lasers (VCSELs) in Co-Packaged Optics (CPO) for high-speed applications, but it requires careful optimization of Gold (Au) bumps bonding parameters to address thermal management, ensure efficient optical coupling, and enhance device performance. This study investigates the packaging performance of a VCSEL array integrated onto silicon interposers using flip-chip bonding with Au bumps under various bonding conditions. Emphasis is placed on the effects of bump flattening and alignment accuracy on bonding quality and overall device performance. Au bumps with flattened surfaces and optimized misalignment were analyzed through SEM imaging, luminescence testing, and L-I-V (Light–Current–Voltage) characterization. Results show that flattening Au bumps and a small misalignment between the two Au bumps significantly improve surface planarity and enhance both electrical and optical performance. Finite Element Analysis (FEA) simulations are also conducted to compare with experimental results and provide further insight into stress distribution and structural behavior. The findings underscore the importance of bump flattening and precise alignment in ensuring mechanical reliability and achieving optimal performance in VCSEL-based optical interconnects for next-generation CPO systems.