Effects of plasma surface treatment and surface roughness on Cu–Cu bonding strength

In this study, Ar plasma surface activation and two planarization methods, electropolishing (EP) and chemical-mechanical polishing (CMP), were systematically investigated and correlated with the bonding strengths of highly <111>-oriented nanotwinned Cu (NT-Cu) bump-to-film joints. Roughness evolution, wettability, and surface chemical states were characterized to assess the effects of planarization and subsequent plasma activation on thermal compression bonding (TCB) quality and shear strength. Topographical analysis showed that CMP produces more surface uniformity and a smoother surface than EP. Specifically, the surface roughness ( R q ) of NT-Cu bumps decreased from 45.8 nm (EP) and 29.6 nm (CMP) to 30.6 nm (EP-Ar) and 19.7 nm (CMP-Ar) after Ar plasma treatment, while the corresponding film surfaces exhibited Rq values of 4.71 nm (EP) and 2.66 nm (CMP), which slightly increased to 6.91 nm (EP-Ar) and 3.00 nm (CMP-Ar) due to mild bombardment. The higher residual roughness of EP-processed surfaces leads to inferior interfacial contact and a higher tendency for seam and void formation, resulting in lower bonding strength. X-ray photoelectron spectroscopy further revealed the presence of Cu−BTA (Cu−benzotriazole) residues on the Cu surface after CMP, indirectly protecting the Cu surface from further oxidation. Subsequent Ar plasma treatment removed these residues and further enhanced surface cleanliness and reactivity. Shear strength tests on bump-to-film joints demonstrated the following order: CMP with Ar plasma treatment (CMP-Ar) had the highest strength, followed by EP with Ar plasma treatment (EP-Ar), CMP without Ar plasma treatment (CMP-ini), and finally EP without Ar plasma activation (EP-ini).