Reductive CH 4 Plasma-Induced Modification of Cu Surfaces for Low-Temperature Hybrid Bonding

As artificial intelligence (AI) and high-performance computing demand greater parallel data processing, next-generation memory architectures must also evolve. High Bandwidth Memory (HBM) supports this with its 3D-stacked structure and wide I/O interface, but further improvements in power efficiency and thermal management are still required. In addition, microbumps currently used for chip stacking face scalability limitations. To address these challenges, hybrid copper (Cu) bonding has been proposed as a promising solution. This study presents a surface treatment strategy for low-temperature Cu-to-Cu direct bonding using CH4-based reductive plasma: (1) native Cu oxides were removed, and (2) re-oxidation was suppressed by forming a hydride carbon passivation layer. A design of experiments (DOE) approach was used to optimize ICP (inductively coupled plasma) power, CH4flow rate, and pressure. Surface analysis was performed using XPS, TOF-SIMS, TEM, and sheet resistance measurements, while post-bonding evaluation included TEM, SEM, and shear strength analysis. The results demonstrate that the energy of radicals and ions plays a decisive role in Cu oxide reduction, contributing to effective surface activation. The resulting passivation layer was confirmed to be a hydrogenated amorphous carbon (a-C:H) film rich in C, CH, and C2H species. Bonding was conducted at 260 °C and 15 MPa for 1 hour, followed by post-annealing at 200 °C. TEM analysis revealed a void-free and oxygen-free bonding interface with the presence of carbon, suggesting that the carbon layer acted as a passivating layer. These findings confirm that reductuve CH4plasma treatment is highly effective for enabling low-temperature Cu bonding, and highlight the strong potential of this approach for next-generation hybrid bonding technologies.