Electrochemical Plating of Metastable Fine Grain Cu for Cu-Cu Hybrid Bonding Applications

Hybrid bonding has emerged as a promising technique for assembling microelectronic devices by integrating various materials with enhanced electrical and thermal performance. Metastable fine grain copper (Cu) has garnered attention as an alternative material for hybrid bonding due to its rapid atomic diffusion at relatively lower bonding temperatures and its compatibility with existing processes. This paper describes the methods for fabricating metastable fine grain Cu and examines how its microstructure evolves through self-annealing and medium-to-high temperature annealing processes, specifically within the 150C to 280C range, applied to patterned wafers.Hybrid bonding, which occurs without external force, leverages the thermal expansion of Cu during elevated temperature annealing to facilitate Cu interdiffusion. The utilization of fine-grain Cu in low thermal budget hybrid bonding capitalizes the energy released during grain growth to enhance bonding strength. This energy derives from changes in grain boundary energy and stress associated with grain growth. The key is to design metastable fine-grain Cu that maintains its fine-grain structure at room temperature for several weeks, while allowing for grain growth at bonding temperatures, typically around 250C.To ensure that metastable fine-grain Cu remains stable at room temperature for several weeks, we have innovated the electroplating process to produce fine-grain Cu with a reduced driving force for grain growth during room temperature self-annealing. Additionally, the Cu can maintain stability during medium temperature annealing at 150C or 170C for about an hour while allowing grain growth at temperatures of 250C or higher. We also observed significant variations in grain growth depending on factors such as via size, aspect ratio, and annealing temperature.Furthermore, we analyze the mechanical properties of metastable fine-grain Cu films, including hardness, residual stress, and tensile strength. Since thermal expansion is crucial for Cu-Cu hybrid bonding, we investigate the thermal expansion characteristics of metastable fine-grain Cu and compare them with normal polycrystalline Cu. We also explore mechanisms for controlling grain growth. Our objective is to provide valuable insights into the complex dynamics of Cu microstructure evolution and demonstrate its potential for high-performance Cu-Cu hybrid bonding applications.