Real-Time TEM Imaging of the Formation of Crystalline Nanoscale Gaps

We present real-time transmission electron microscopy of nanogap formation by feedback controlled electromigration that reveals a remarkable degree of crystalline order. Crystal facets appear during feedback controlled electromigration indicating a layer-by-layer, highly reproducible electromigration process avoiding thermal runaway and melting. These measurements provide insight into the electromigration induced failure mechanism in sub-20 nm size interconnects, indicating that the current density at failure increases as the width decreases to approximately 1 nm.

Figure 1

Formation of a crystalline nanogap. (a) TEM image of a nanogap forming during FCE (gold is dark). Electromigration occurs in the region within the dotted square. (b) Image at a later time showing crystal faceting and thinning in the lower part of the nanogap, where electromigration is occurring. The arrow indicates a step edge. (c) Crystal planes form angles of 120° in the nanogap region, indicative of electromigration along the $⟨110⟩$ directions of $\{111\}$ oriented gold. (d) Electromigration has begun to occur along the top edge, where crystal faceting is now evident. (e) The lead is now pinched down to about 1 nm in size, corresponding to approximately 10 atomic channels of conduction. (f) The $G\mathrm{\text{−}}V$ characteristic of the sample with letters denoting the conductance for the corresponding images in the parts (a)–(e) of the figure.

Figure 2

Unzipping model of evolution of a nanogap during FCE. (a) Starting with the initial wire, an edge atom is thermally excited. (b) This atom is easily transported away by the applied current along the high-mobility $⟨110⟩$ directions. (c) The edge vacancy allows adjacent atoms to easily electromigrate and jump to the edge due to their reduced number of nearest neighbors. (d) The edge is thus unstable once the unzipping of a layer begins.

Figure 3

Reversible electromigration. (a) Initial bowtie structure. (b) Current is applied so that electrons flow from left to right, indicated by the arrow. (c) The lead begins to form a faceted void. Electromigration is ceased at this point when the electromigrated void is clearly the narrowest region of the wire. (d) The applied voltage is reversed and the void begins to fill back in with gold. (e) This is continued to the point where the void is completely filled in. (f) Further electromigration in this direction yields a hillock at the site of the initial void while a faceted void starts to form at the opposite side of the bowtie structure.

Figure 4

Width dependence of $I^{*}$ and $G_n$. (a) $I^{*}$ is plotted against the corresponding neck width of the constrictions for the sample in Fig. 1. The solid (blue) line represents a power-law fit to $I^{*}\sim W^{1/2}$ and the dashed (red) line represents $I^{*}\sim W$. (b) $G_n$ plotted against neck width where the solid (blue) line represents a power-law fit to $G_n\sim W$ and the dashed (red) line represents $G_n\sim W^2$.