Non-TCB Process Cu/SiO₂ Hybrid Bonding Using Plasma-free Hydrophilicity Enhancement with NaOH for Chip-to-wafer Bonding

We have developed a novel process highly suitable for chip-to-wafer bonding, where 0.5 Μ NaOH is used instead of plasma to achieve better oxide bonding quality. This successfully completes the robust Cu/SiO₂ hybrid bonding with only 1 min of prebonding at room temperature followed by post-bonding annealing without external pressure. In this investigation, we validated the considerable enhancement of SiO₂ surface hydrophilicity after immersion in NaOH. Additionally, surface analysis by atomic force microscopy confirmed that the roughness of the Cu/SiO₂ hybrid surface was unaffected. Through NaOH, we successfully achieved room-temperature bonding of SiO₂ blanket films in just 5 min, with a bonding strength of up to 19 MPa and 99 % bonding area integrity. Such high-strength SiO₂ bonding can withstand the tensile stress induced during the post-bonding annealing, enabling the realization of non-TCB (thermal compression bonding) Cu/SiO₂ hybrid bonding. From the cross-sectional scanning electron microscopy images, it is evident that there are no significant cracks at the SiO₂-SiO₂ and Cu-Cu bonding interfaces. Moreover, transmission electron microscopy (TEM) analysis reveals phenomena of grain growth, with no obvious chemical residuals or oxide formations observed by TEM energy dispersive X-ray. Very low specific contact resistivity of 3.2 × 10^-10 Ω•cm² was detected using 4-point probes.