Effect of conductivity on the electromigration-induced morphological evolution of islands with high symmetries of surface diffusional anisotropy

We report on the electromigration-induced morphological evolution of islands (vacancies, precipitates, and homoepitaxial adatom clusters) using a phase-field method with high symmetries of surface diffusional anisotropy. The analysis emphasizes on islands migrating in the {100} and {111} planes of the face-centered-cubic crystal, which resembles fourfold and sixfold symmetries, respectively. The numerical results intend to elaborate on the role of conductivity contrast between the island and the matrix and the misorientation of the fast diffusion direction with respect to the applied electric field on the morphological evolution. Based on numerical results, a morphological diagram is constructed in the plane of misorientation angle and conductivity contrast delineating a rich variety of morphologies, which includes steady-state, time-periodic, zigzag oscillations, and island breakup. While the shape of the island is primarily dictated by the conductivity contrast, the migration modes depend on the misorientation. The various migration modes are further distinguished based on the shape of the island such as a faceted wedge or seahorse morphology, an oscillatory characteristic such as standing wave or traveling wave time-periodic oscillations, and different breakup features. The steady-state kinetics obtained from the fourfold and sixfold symmetries are critically compared with the twofold symmetry, isotropic analytical, and numerical findings. Our result suggests that the steady-state velocity decreases with the symmetry fold of the island. Furthermore, the influence of variation in conductivity contrast and misorientation on kinetics in the time-periodic oscillations are discussed. Finally, the numerically obtained stable facets are compared with the analytically derived orientations. The observed results have direct repercussions in terms of the fabrication of nanopatterns and the performance of thin-film interconnects.