Optimizing Cu-Cu bonding quality in nanotwinned Cu films through plasma-induced surface roughening

• Ar plasma tunes surface roughness and preserves (111)-oriented surface. • Greater roughness increases stress gradients and promotes early Cu-Cu bonding. • Plan view TEM shows smaller and more uniform voids for rough pairs at the early stage. • At later stages roughness improves interfacial uniformity and reduces void fraction. This study explores low-temperature Cu-Cu bonding using highly <111>-oriented nanotwinned copper (NT-Cu) films with various surface roughness. Ar plasma treatment was applied to modulate surface roughness with Rq = 1.6, 2.5, 5.2, and 10.9 nm, measured by atomic force microscopy (AFM). The specimens were bonded under varying conditions at 200 °C under 10∼30 MPa for 10∼60 min. Three bonding configurations—flat-flat, flat-rough, and rough-rough—were examined. Cross-sectional focused ion beam (FIB) and plan-view transmission electron microscope (TEM) revealed that rougher surfaces promoted higher local stress gradients and diffusion, resulting in ∼60% void area reduction after bonding at 200 °C for 1 h. These results demonstrate that plasma-induced roughening improves Cu-Cu bonding efficiency under limited thermal budgets and offers a deeper understanding on the Cu-Cu bonding. The above results are counterintuitive, and the possible mechanisms will be discussed in this article.