A solid state process to obtain high mechanical strength in Cu-to-Cu joints by surface creep on (111)-oriented nanotwins Cu

Abstract

In 3D IC integration, a critical demand of interfacial joints in high-end devices is ultra-fine pitch, below 2 μm. The pitch cannot be fabricated by using solder joints because of its low melting point. Microelectronic industry prefers a solid state process, but the processing temperature cannot be too high, preferable near 250 °C. Thus, Cu-to-Cu direct bonding has attracted much attention. We have bonded the highly (111)-oriented Cu microbumps to highly (111)-oriented Cu films as well as to randomly-oriented Cu films for comparison, at temperatures from 200 to 350 °C. We examined the microstructures of bonded interfaces and conducted shear tests to evaluate the mechanical strength of the Cu joints. The bonded interfaces for the (111) Cu microbumps to (111) Cu films had fewer voids than those bonded to randomly oriented Cu films. In addition, the bonding strength of the latter is much less than the former. We propose that the fast surface diffusivity on the (111) surfaces play a critical role on surface creep, which increases the bonding strength of the Cu joint. A surface creep model and its creep rate are presented, and a comparison to the experimental data has been made; the agreement is reasonable.