Oxidation on Nano-Twinned and Polycrystalline Cu Films
摘要
With continued scaling and functional integration in semiconductor technology, Cu has become indispensable in advanced packaging. However, Cu surfaces are highly susceptible to oxidation in ambient environments, and the resulting oxide layers act as barriers during Cu–Cu bonding. Nano-twinned (nt) Cu and polycrystalline (poly) Cu films were prepared by electrodeposition, where the nt-Cu film exhibits a columnar structure with a high density of coherent (111) twin boundaries, while the poly-Cu film shows a (110)-preferred orientation. Surface oxidation was conducted at 300°C for 5–20 min. The calculated oxidation rates for the poly-Cu and nt-Cu films are 19.8 nm/min and 15.5 nm/min, respectively. The faster oxidation of poly-Cu is attributed to the higher surface energy and dense dangling bonds on the (110) plane, which enhance oxygen adsorption, while the low-energy (111) surface suppresses oxidation. XPS analysis reveals comparable Cu2+ and Cu+ fractions in poly-Cu, whereas the nt-Cu oxide layer is dominated by Cu+, indicating the reduced Cu atomic flux associated with dense nano-twin boundaries. Voids at the oxide/Cu interface increase with oxidation time due to oxidation-driven outward Cu diffusion, while the minimal oxidation observed within the voids suggests that oxidation is dominated by reactions at the outermost oxide surface.