Supporting data for “Microstructure and orientation control of electro-deposited copper and its application in advanced electronic packaging”

In advanced IC packaging, chip stacking has emerged as a promising technology to overcome the physical limitations imposed by Moore's Law while continuing to achieve higher interconnect density and compact form factors. The electroplated copper used for connections in these packages has varying demands for structural stability across different procedures. For instance, in redistribution layer (RDL) applications, the width and pitch of RDLs for device connections continue to shrink, with some high-end RDLs potentially featuring lines as narrow as 2 μm. This places greater demands on the stability of copper structures to resist thermal migration and electromigration. To satisfy these needs, a composite Cu structure that combines fine grains and nanotwins was successfully developed, providing enhanced super filling capabilities and improved structural stability. This structure can effectively resist surface damage even after extended periods, offering a promising solution for achieving finer RDL in advanced packaging technologies. Furthermore, it exhibits an EM lifetime 2.9 times longer than its coarse-grain RDLs counterpart.

Conversely, for bonding applications that connect vertical dies, it is essential to use electroplated copper with lower structural stability to minimize the thermal budget of Cu-Cu direct bonding and decrease potential damage to temperature-sensitive device. To satisfy these needs, a unique composite Cu structure is developed by incorporating a nanograin structure into the (111)-oriented coherent nanotwinned copper. This metastable composite structure remains stable at room temperature, ensuring its durability throughout long fabrication processes after electroplating, and then undergoes grain growth upon exposure to higher temperatures during bonding. The nanograin-enriched area of the composite copper undergoes substantial grain growth and facilitates atomic movement with adjacent nanotwins, enabling the growth of grains across the bonding interface to enhance the bonding quality. Successful Cu-Cu direct bonding can be achieved at a low thermal budget of 170 °C for 30 minutes, resulting in improved mechanical and electrical performance as evaluated by mechanical shear tests, electromigration tests, and thermal cycling tests.

To further reduced the thermal budget of bonding process, then a super unstable nanograin copper structure specifically aims for Cu-Cu bonding at temperatures below 100 °C, or even at room temperature. This super unstable nanograin copper structure exhibits rapid self-annealing behavior, undergoing complete recrystallization at room temperature within 250 minutes or at 60°C within just 5 minutes. The driving force for grain growth can be attributed to the large excess energy stored in the high-angle grain boundaries (HAGBs) and dislocations. The calculated activation energy for the coarsening of super unstable copper grains is approximately 0.57 eV/atom, which is significantly lower than those reported for nanograin coppers using the same differential scanning calorimetry method.