Excellent Oxidation Resistance and Solder Wettability of (111)-Oriented Nanotwinned Cu

In the advanced-packaging industry, (111)-oriented nanotwinned copper ((111)nt-Cu) offers several advantages over common randomly oriented equiaxed polycrystalline Cu (C-Cu), including high strength, excellent elongation, and promising electrical conductivity. In recent years, (111)nt-Cu has shown potential for becoming a C-Cu replacement in under-bump metallization and redistribution layer applications. This shift is attributed to the escalating demand for thermal stability and enhanced electrical and mechanical performances of Cu materials amid the rapid transition toward three-dimensional electronic packaging. This study proposed that (111)nt-Cu exhibits better oxidation resistance than C-Cu after aging in air at 250 degrees C. The thickness and composition of (111)nt-Cu and C-Cu oxide layers were analyzed respectively via TEM and XPS. Various grain boundaries on the surface of the prepared (111)ntCu and C-Cu substrates were evaluated using EBSD and FIB techniques. The morphology at the interface between the two Cu substrates and their oxide layers was characterized using SEM. In addition, the solderability of the oxidized layers was assessed by measuring the wetting angles and spreading areas of the involved Sn-Cu joint structures. Results show that when the oxidation time was 9 min, the thickness of the (111)nt-Cu oxide layer was 43.2% lesser than that of the C-Cu oxide layer. After 12 min of oxidation, the contact angle between Sn and oxidized (111)nt-Cu was 26.7% smaller than that between Sn and oxidized C-Cu, while the spreading area of Sn on (111)nt-Cu was 24.6% larger than that of Sn on C-Cu. After oxidation, the surface layers of both Cu substrates comprised CuO and Cu2O nanocrystals coexisting within the same layer. Because oxidation is closely related to the diffusion of Cu atoms through grain boundaries, the grain boundaries of both Cu substrates were investigated in the natural-growth direction. The results show that compared to C-Cu, (111)nt-Cu has a lower surface energy and smaller area fraction of high angle grain boundaries, effectively limiting the outward-diffusion rate of Cu atoms.