A reliable copper-free wafer level hybrid bonding technology for high-performance medical imaging sensors

Current 3D integrated devices based on copper hybrid bonding are only integrating dual-damascene CMOS processes for interconnection.. In this study, we have developed a Titanium/Oxide (Ti/SiO
₂
) hybrid bonding technology suitable for 200 mm non-copper technology platforms featuring aluminum back-ends. A combination of material stack, CMP parameters and design rules enabled us to obtain defect-free bond interface across the wafer. Scanning acoustic microscopy, FIB-SEM and TEM cross-sections demonstrated a perfect SiO
₂
/SiO
₂
bonding as well as excellent Ti/Ti connections for Ti pads as small as 3×3 μm
²
. Moreover, we designed an electrical test vehicle including a Ti/SiO
₂
hybrid-bonding interface, with W vias and AlCu metal interconnects. Despite the integration stack, the bonding interface remains defect-free with contact pads ranging from 10×10 μm
²
down to 3×3 μm
²
. The 3D interconnect showed a contact resistance of 1 Ω and a parasitic capacitance of less than 2 fF. Finally, we report on the process reliability by means of thermal cycling and high-temperature storage, which confirmed the robustness of this innovative Ti/SiO
₂
fine pitch interconnection.