Prior-to-bond annealing effects on the diamond-to-copper heterogeneous integration using silver–indium multilayer structure

In the present study, we have successfully demonstrated the heterogeneous integration of chemical vapor deposition (CVD) diamond and copper (Cu), using a fluxless silver–indium (Ag-In) bonding system. CVD diamond heat spreaders and Cu heat sinks are crucial components for efficient heat dissipation in high-power electronics and photonics. The two components were bonded by a multi-layer Ag-In structure. Microstructure of joints revealed isolated voids, caused by undersupply of molten phase (In) during bonding. Thus, the prior-to-bond Ag annealing process was introduced, in order to resolve the undersupply of the molten phase. Annealing the Ag reduced the grain boundary density and subsequently slowed the diffusion of In into Ag. The prior-to-bond annealing of Ag was conducted in air and in vacuum environment and completely eliminated the isolated voids at the original bonding interface, stemming from the undersupply of molten phase. However, unforeseen adverse effects of the prior-to-bond annealing process were also discovered for the first time, which degraded the shear strength of the Ag-In joints. The underlying degradation mechanisms were thoroughly investigated with scanning electron microscopy, energy-dispersive X-ray spectroscopy, focused ion beam, and X-ray photoelectron spectroscopy. Microstructural studies showed that the formation of metallic oxides and metallic sulfides was responsible for the mechanical degradation of the joint for air- and vacuum-annealed specimens, respectively. The underlying reasons for the formation of such mechanically adverse compounds were thoroughly discussed. With the knowledge of the degradation mechanism, this work will pave the path for the future technical advancements of the Ag-In bonding method, which can be utilized for the integration of CVD diamond into the package, thereby improving the thermal performance and the reliability of the high-power electronics and photonics.