Interfacial Characterization of Low-Temperature Cu-to-Cu Direct Bonding with Chemical Mechanical Planarized Nanotwinned Cu Films.

Copper-to-copper (Cu-to-Cu) direct bonding is a promising approach to replace traditional solder joints in three-dimensional integrated circuits (3D ICs) packaging. It has been commonly conducted at a temperature over 300 °C, which is detrimental to integrated electronic devices. In this study, highly (111)-oriented nanotwinned (nt) Cu films were fabricated and polished using chemical mechanical planarization (CMP) and electropolishing. We successfully bonded and remained columnar nt-Cu microstructure at a low temperature of 150 °C thanks to the rapid diffusion of Cu on (111) surface. We employed a new microstructural method to characterize quantitatively the interfacial bonding quality using cross-sectional and plan-view microstructural analyses. We discovered that CMP nt-Cu bonding quality was greater than that of electropolished nt-Cu ones. The CMP nt-Cu films possessed extremely low surface roughness and were virtually free of pre-existing interface voids. Thus, the bonding time of such CMP nt-Cu films could be significantly shortened to 10 min. We expect that these findings may offer a pathway to reduce the thermal budget and manufacturing cost of the current 3D ICs packaging technology.