(223i) Current-Driven Nanowire Formation and Nanopatterning on Crystalline Conducting Substrate Surfaces

The ability to form and precisely manipulate nanoscale features by controlling macroscopic forces is essential to advancing nanotechnology. For example, a macroscopic force in the form of an externally applied electric field can be used to drive atomic motion through the mass transport phenomenon of electromigration providing a paradigm shift in surface engineering and nanofabrication based on physical patterning.

Toward this end, in this presentation, we report a simulation study on formation of nanowires with precisely controlled widths, starting from single-layer conducting islands on face-centered cubic (fcc) crystalline conducting substrates under the controlled action of macroscopic forcing provided by an externally applied electric field that drives island edge electromigration. Our continuum-scale, fully nonlinear driven island evolution model with diffusional mass transport limited to the island edge accounts for edge diffusional anisotropy and island coalescence and breakup; the model is validated by comparison of its predictions with experimental results for Ag islands on Ag substrates reported in the literature. Numerical simulations based on this experimentally validated model and supported by linear stability theory show that large-size islands undergo a current-induced fingering instability, leading to nanowire formation after finger growth. Depending on the substrate surface crystallographic orientation, necking instabilities after fingering lead to the formation of multiple parallel nanowires per island. In all cases, the axis of the formed nanowires is aligned with the direction of the externally applied electric field. The nanowires have constant widths, on the order of 10 nm, which can be tuned by controlling the externally applied electric field strength. Achieving parallel arrays of ~10-nm-wide nanowires, dimensions barely accessible even by e-beam lithography, and demonstrating the ability to control their direction has important implications for developing future nanoelectronic patterning techniques applicable, e.g., to the interconnect structures of future electronic chips.

In addition, we demonstrate based on direct dynamical simulations that under specific conditions, and over a range of edge diffusional anisotropy parameters and nanowire edge orientation, the nanowire straight edge becomes unstable under the action of the externally applied electric field. This current-induced nanowire edge morphological instability leads to breakup of the nanowire into smaller-size daughter islands of practically uniform size and spacing to form regular patterns of nanoscale-size daughter islands. We have conducted a comprehensive parametric study to provide guidelines for generating such regular nanopatterns, which can be classified broadly into two categories depending on the geometrical orientation of the daughter islands in the regular array: aligned along a straight line or aligned along two intersecting straight lines to form a â??Vâ??-shape. Moreover, using a linear stability theory we explain the origin of this nanowire straight edge instability; the theory is successfully validated by comparing its predictions for the number of daughter islands formed and the characteristic spacing between the islands with the results of the dynamical simulations. Our findings have direct implications for the development of novel external-field-driven physical nanopatterning techniques toward device nanofabrication.