Atomistic simulation of hillock growth

Abstract

This paper explores the mechanisms of hillock and whisker growth in stressed polycrystalline films by molecular dynamics simulations. The initial geometry consists of three grains with a triple line aligned perpendicular to a free surface, plus a fourth pyramidal-shaped grain implanted between the triple line and the surface. This simulated grain geometry corresponds to that observed in experiments during hillock and whisker growth, with the fourth grain serving as a seed for hillock growth. The simulations, performed under an applied in-plane biaxial compression, reveal an upward motion and growth of the seed grain. The growth occurs by stress-driven grain boundary diffusion from below the seed grain onto some of its internal faces. Accretion of atoms to those faces pushes the seed grain upwards and sideways. The different diffusion and accretion rates at different boundaries also give rise to internal stresses, which can be partially accommodated by grain boundary motion coupled to shear deformation. The hillock growth is countered by surface diffusion, which can slow the growth or even suppress it completely. Other mechanisms involved in hillock growth are also discussed.