Mechanistic Study of Low-Temperature Plasma-Activated InP/Al₂O₃/SOI Direct Wafer Bonding for High Bonding Energy

Silicon-based photonic integrated circuits (PICs) are the core hardware for advanced optoelectronic integration, yet silicon's indirect band gap limits the realization of high-performance on-chip light sources. Heterogeneous integration of direct band gap indium phosphide (InP) on Si thus provides a critical pathway to high-performance PIC with monolithically integrated light sources. However, the interfacial chemical mechanisms governing the strength and reliability of plasma-activated InP/silicon-on-insulator (SOI) direct bonding remain poorly understood. In this work, we demonstrate that high-strength and high-reliability InP/SOI bonding is dominated by two key interfacial factors: preservation of surface reactive hydroxyl groups and suppression of oxide-driven interfacial diffusion on InP. We elucidate how these processes determine bonding interface formation and failure via Water Contact Angle (WCA) measurements, molecular dynamics (MD) simulations, in situ X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). We find that Al₂O₃ outperforms SiO₂ as the interlayer: stable Al-OH formed after activation inhibits hydroxyl self-condensation and retains higher reactive hydroxyl density for interfacial bonding, while Si-OH self-condensation on activated SiO₂ depletes reactive hydroxyls and weakens heterogeneous bonding. We further show that N₂ plasma activation forms a passivation layer on InP to suppress surface oxidation and interfacial interdiffusion, enabling smooth, high-quality bonding interfaces. In contrast, O₂ plasma activation causes pronounced interfacial diffusion and brittle In₂O₃ formation, triggering interfacial embrittlement and reduced bonding strength. Guided by these mechanistic insights, we achieved a maximum bonding strength of 12.5 MPa. This work identifies the key interfacial chemical mechanisms of plasma-activated InP/SOI bonding, providing a solid mechanistic foundation and a practical route for high-reliability heterogeneous integration in high-speed, low-power optical interconnects.