Asymmetric interfacial reaction governed by twin orientation in hybrid nanotwinned copper interconnects

The asymmetric interfacial reaction in (111) nanotwinned Cu (nt-Cu)/Sn/(110) nt-Cu interconnects was investigated using in-situ synchrotron radiation imaging and electron backscatter diffraction (EBSD). Real-time imaging reveals that the growth rate of interfacial intermetallic compounds (IMCs) at the (111) nt-Cu interface reached 1.14 μm/min, approximately eight times higher than that at the (110) nt-Cu interface (0.14 μm/min). This pronounced asymmetry arises from the distinct twin-boundary configurations relative to the principal diffusion direction. The horizontally oriented twins in (111) nt-Cu intersect vertical grain boundaries at high frequency, generating abundant triple junctions that reduce grain-boundary energy and lower the effective Cu diffusivity from the substrate. In contrast, the near-vertically oriented twins in (110) nt-Cu produce far fewer triple junctions, leaving grain-boundary diffusion channels largely open. This difference establishes a directional Cu concentration gradient across the molten solder, quantified by a concentration gradient driven flux that drives preferential IMC thickening at the (111) nt-Cu interface. In the as-soldered state, EBSD analysis shows that Cu 6 Sn 5 grains at the (111) exhibit a strong {11 2 ‾ 0} texture with an epitaxial relationship {11 2 ‾ 0}Cu 6 Sn 5 ||{111} Cu , whereas those at the (110) nt-Cu interface exhibit a preferred orientation in which the sample normal (RD) is clustered approximately 60° away from <0001> η (i.e., approximately 30° from <11 2 ‾ 0> η ). Upon prolonged soldering, the initially preferred crystallographic orientation of Cu 6 Sn 5 at both interfaces became random, driven by rapid Cu 3 Sn thickening that disrupts epitaxial matching at the (111) interface, and by transverse ripening that promotes coalescence of preferentially oriented Cu 6 Sn 5 grains at the (110) nt-Cu interface. These findings demonstrate that the relative orientation of coherent twin boundaries with respect to the principal diffusion direction is a critical microstructural parameter governing interfacial reaction kinetics, and they provide a mechanistic basis for the rational design of orientation-engineered under bump metallization structures in advanced electronic packaging.