Enabling 3D Heterogeneous Integration of Silicon Photonics Chiplet Through UBM-Free Au-SnAg Bonding for 2Tb/s Optical Interconnects

As Moore's Law slows with chip process nodes approaching physical limits, advanced packaging like flip-chip technology plays a very important role, particularly in the field of optical communication device packaging. Through the method of bonding gold bumps on the aluminum pad, the aluminum pad and the SnAg-based solder are indirectly interconnected. This paper presents a low-cost and high-performance 3D heterogeneous integration approach for the electronic integrated circuit (EIC) and the photonic integrated circuit (PIC) by eliminating the under bump metallization (UBM) structure, which is conventionally required for photonic-electronic interconnects. The proposed method employs ultrasonic bonding and thermocompression bonding to fabricate gold bumps on aluminum (Al) pads of the photonic integrated circuit (PIC) , followed by reflow soldering to achieve reliable interconnections between the gold bumps and SnAg solder cap of the electronic integrated circuit (EIC)[1]. By applying pressure through a plate, the gold bump surface is flattened, thereby enhancing the contact interface between the gold bump and the SnAg solder cap. Radio frequency (RF) simulation and link analysis demonstrate that the UBM-free architecture achieves ≥200 GHz bandwidth, with a 38.5% lower insertion loss (-0.32 dB vs. -0.52 dB for UBM-based) at 200 GHz. A 3D-integrated silicon photonics chiplet based on this technology achieves a single-channel data rate of 256 Gb/s and an aggregate unidirectional bandwidth of 2 Tb/s, addressing the challenges of multi-project wafer (MPW) environments by eliminating costly UBM processes.