Morphological Microstructure Characterization and Optimisation of Nanocrystalline Copper Deposition for Fine-Pitch Hybrid Bonding Cu/SiO2 at Low Temperature

Wafer-to-wafer hybrid bonding technology is a key enabler to achieve fine-pitch 3D integration, allowing shorter electrical paths, lowest signal loss, higher performances, and increased reliability. In the context of 3D integration, achieving hybrid bonding of copper pads at reduced thermal budgets, typically below 200°C, remains challenging. Optimizing the copper microstructure is an enabler to lower thermal budgets, for applications such as Phase Change Memory or Quantum Film. In the literature, two promising copper microstructures are studied to address the low-temperature challenges: nanocrystalline copper (NC-Cu) also known as “fine grain copper” and nanotwinned copper (NT-Cu). The latter offers advantages such as effective diffusion, an optimized coefficient of thermal expansion, maintaining low resistivity and controlling copper oxidation process. However, due to the smaller dimensions of the fine pitch structures nanocrystalline copper are more easily developed compared to nanotwinned copper. In this study, several electroplating conditions were tested to optimize the stability of copper deposits over time and assess production feasibility. Electroplating was performed using a specific waveform on a 300 mm wafer tool with a commercial bath. Key electrodeposition conditions were identified to improve the kinetic control of nanocrystalline copper self-annealing after several days of deposition. The filling process of nanocrystalline copper for different fine pitch structures $(0.6 \mu \mathrm{m} \text{to} 10 \mu \mathrm{m})$ has also been optimized to obtain nanograins without voids and to achieve hybrid bonding at low temperature.