Study of Low Temperature Cu-to-Cu Bonding Using Reducing Plasma

This study investigates the effect of CH4 plasma treatment on the quality of Cu-to-Cu bonding. An 8-inch notch-type Si wafer with a 700 nm SiO₂ layer is utilized. A Ti adhesion layer of 50 nm and a 1 μm Cu thin film are subsequently deposited onto the Si/SiO₂ wafer using DC magnetron sputtering. Following the Ti/Cu deposition, CH₄ plasma treatment is applied using inductively coupled plasma chemical vapor deposition (ICP-CVD). The influence of CH₄ on Cu surface is optimized with power that affect plasma concentration, and gas flow ratio (Ar:CH₄) that can directly affect reaction mechanisms or surface changes. Cu-to-Cu bonding is performed at 260°C for 1 hour, followed by annealing at 200°C for 1 hour. Oxides and contaminants on the Cu surface are removed and the surface roughness decreases as plasma-treatment time increases, as confirmed by X-ray photoelectron spectroscopy(XPS) analysis and atomic force microscopy(AFM) measurement. Bonding quality is evaluated using SAT, FE-SEM, TEM, which shows that the plasma-treated specimen has fewer voids at the bonding interface and more Cu diffusion. The results show that CH₄ plasma treatment enhances Cu-toCu bonding quality.