Residual stress effect governing electromigration-based free-standing metallic micro/nanowire growth behavior

Abstract

In this study, the effect of residual stress in a film on the growth behavior of a free-standing metallic micro/nanowire due to electromigration (EM) is examined. The growth of a wire is accompanied by atomic diffusion, accumulation of atoms, and release of compressive EM-induced localized hydrostatic stress due to the accumulation of atoms. Hence, the growth of the wire dominantly depends on the EM-induced localized stress caused by the accumulation of atoms. Because rigid passivation generates a strong localized stress field in the metallic interconnect, with greater accumulation of atoms, the EM-induced localized stress state for wire growth is influenced by passivation conditions, including the thickness and residual stress associated with passivation. Two samples with different passivation thicknesses, resulting in different levels of residual stress, were used to elucidate the influence of passivation conditions on the growth performance of Al microwires. The growth rate was experimentally measured. An x-ray diffraction system was used to obtain the value of residual stress in passivation, demonstrating that a higher absolute value of compressive residual stress results in a lower growth rate. In contrast, a lower absolute value increases the growth rate of the wire and can decrease the delamination risk of the topmost passivation, deposited by sputtering. Contrarily, a passivation that is too thin, resulting in a lower absolute value of compressive stress, increases the risk of passivation crack due to the accumulation of atoms by EM. A suitable passivation thickness for a desired wire growth must be determined based on this finding.