Effect of CH₄ Plasma Pretreatment on Cu-to-Cu Bonding Applications

Cu-to-Cu direct bonding offers significant advantages for advanced 3D integration, including reduced power consumption and a lower thermal budget. In this study, a reductive plasma-based Cu-to-Cu direct bonding method is proposed for stacked high bandwidth memory (HBM) applications. While conventional Ar/H₂ (5%) forming gas plasma is commonly used for Cu oxide reduction, its low hydrogen radical and ion density results in extended treatment times and elevated temperatures. To address these limitations, an Ar/CH₄-based plasma process is introduced, providing a higher density of hydrogen radicals and ions, thereby enhancing the reduction efficiency under low temperature conditions.An 8-inch silicon wafer with a Ti/Cu metal layers was fabricated using DC magnetron sputtering, followed by Cu/barrier chemical mechanical polishing (CMP) and CH₄ plasma treatment via inductively coupled plasma chemical vapor deposition (ICP-CVD). The Cu-to-Cu bonding process was carried out for 1 hour at 260°C, and this was followed by a subsequent annealing step at 200°C to promote Cu-Cu bonding through grain boundary diffusion. Plasma process parameters including power, gas flow rate, and pressure were optimized for Cu oxide removal and surface passivation.X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) analysis confirm that Ar/CH₄ plasma effectively reduces Cu oxides. Furthermore, a carbonaceous layer with a thickness of 1-3nm was formed on the reduced Cu, acting as a passivation layer to suppress reoxidation. Scanning electron microscopy (SEM) analysis of the post-bonded structure revealed a void-free bonding interface, demonstrating the effectiveness of the proposed method in achieving high-quality Cu-to-Cu direct bonding.