Atomic Migration of Cu on the Surface of Si/Ti/Ni/Cu/Ag Thin Films

To improve the adhesion between nickel (Ni) and silver (Ag) films in Ti/Ni/Ag backside-metallized silicon (Si) wafers, an additional layer of copper (Cu) film was added between the Ni and Ag layers. It was found that high-temperature storage caused the formation of voids in the Ag layer and degraded the whole structure. During high-temperature metallization, the Cu layer fluctuated slightly and formed a wavy layer. With extended heating time, the Ag/Cu became uneven, and some Cu atoms diffused to the surface of the Ag layer. EDX mapping revealed that the Cu layer had split into two distinct layers. Finally, most of the Cu atoms migrated to the surface and the intermediate Cu layer was gradually exhausted. A proposed mechanism is that, as heat was applied to the Ti/Ni/Cu/Ag metal stack, the Ag grain growth initiated void formation in the Ag layer. The Cu atoms, energized by the thermal energy, diffused outward through the Ag channels connecting the voids and eventually spread over the metal stack to form a uniform layer. In this study, it is also evidenced that Cu atomic migration can be prevented by the deposition of a Sn layer on the Ti/Ni/Cu/Ag metallization.