Control of Cu Microstructure and Properties for Low-Temperature Direct Cu-Cu Bonding

3D heterogeneous integration offers significant improvements in terms of input/output density, faster processing speeds, and reduced power consumption. Cu-Cu hybrid bonding is critical for unlocking the full potential of this technology. However, high temperatures associated with the Cu-Cu bonding process have limited the scope of this technology. Two primary strategies have been proposed for modifying Cu microstructure to enable low-temperature bonding capabilities: (111)-oriented, nanotwinned Cu and fine-grained Cu. Herein, we demonstrate the ability to reliably deposit both types of Cu microstructures on blanket and patterned segments by modifying bath additives and plating conditions. To be viable for high-volume manufacturing, functional Cu deposits must be stable for several weeks and persist through the processing steps prior to bonding. In this study, we investigate the stability of Cu deposits with time and through the chemical mechanical polishing (CMP) process. We report the Cu microstructure before and after CMP by x-ray diffraction (XRD), focused ion beam scanning electron microscopy (FIB-SEM), and electron backscattered diffraction (EBSD) analysis. Further, we seek to understand the unique properties of fine-grained Cu that determine its stability under various annealing conditions. To this end, we perform experiments to quantify stress in the deposits, characterize grain boundaries, and measure dopant levels. By understanding the properties associated with Cu grain stability, we can improve our formulations for features with decreasing and mixed critical dimensions associated with next-generation hybrid bonding.