Low-Resistance Cu–Cu Bonding With the Formic Acid-Treated Co Passivation Layer

The Cu–Cu bonding with formic acid-treated Co passivation layer was successfully achieved at 230 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">∘</sup>C under atmospheric pressure, resulting in high bonding strength. TEM and energy-dispersive X-ray spectroscopy (EDS) characterization of the bonding interface has revealed that the reduction of oxides through formic acid treatment (FAT) promoted the merging of Co grains while concurrently minimizing the concentration of Cu in center area. The sample with FAT achieved ultralow contact resistance of 3.58 × 10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-<roman>8</roman></sup> Ω⋅cm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><roman>2</roman></sup> at 250 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">∘</sup>C, lower than that without FAT. We analyzed the reasons for the decrease in contact resistance from the bonding interface and found that FAT reduced the surface roughness and oxides, resulting in better bonding of the Co passivation layer and reduced electron scattering. It was concluded that an ideal bonding interface necessitated minimal electron scattering.