Fabrication and mechanical behavior of (111) oriented nanotwinned Cu-bumps toward Cu-Cu bonding for 3D packaging

Realizing Cu-Cu bonding with (111)-oriented nanotwinned Cu pillar bumps (Cu-bumps) is a key route to overcoming the limitations of Moore’s Law and enabling high-density 3D packaging. In this study, we optimized direct current electrodeposition and produced highly (111)-oriented nanotwinned Cu at a stirring speed of 1100 rpm. We then fabricated Cu-bumps and examined their deformation behavior before and after thermal aging under five bonding cycles with progressively increased compressive loading. In the as-electrodeposited state, the bump deformation was governed by a lamella-thickness-dependent transition between Type-II confined-layer slip and Type-III partial glide. Localized stresses at the bump head activated Type-II slip, thinning lamellae without inducing detwinning, whereas delayed activation of Type-III at the base led to detwinning. In comparison, thermal-aging-induced recovery reduced lattice friction and accelerated Type-III glide, leading to pronounced softening. The softened bump showed a marked reduction in compressive strength, potentially increasing the risk of tilting or bridging in fine-pitch interconnects. These results elucidate the mechanical response of (1   1   1)-oriented nanotwinned Cu bumps under repeated Cu-Cu thermocompression bonding and provide critical insights into identifying and mitigating failure risks in advanced 3D packaging.