Facet-Facet Barrier on Surfaces: Proposal and Experimental Validation
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Facet-Facet Barrier on Surfaces: Proposal and Experimental Validation Hanchen Huang*, Department of Mechanical, Aerospace and Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180; C. H. Woo, H. L. Wei, S. J. Liu, Department of Mechanical Engineering, Hong Kong Polytechnic University, Hong Kong, X. X. Zhang and M. Altman, Department of Physics, Hong Kong University of Science and Technology, Hong Kong, and E. G. Wang, Institute of Physics, Chinese Academy of Science, Beijing. ABSTRACT Surface processing is generally kinetics limited. Migration barriers, both on flat surfaces and near steps, therefore are of crucial importance. In this paper, we describe a kinetic barrier that separates two neighboring facets on surfaces. The contrast of this concept relative to the conventional Ehrlich-Schwoebel barrier is shown through molecular dynamics/statics calculations on copper and aluminum. The effects of this new kinetic barrier are demonstrated through lattice Monte Carlo simulations. The predicted effects are in direct correspondence with validation experiments on copper and silver thin films. INTRODUCTION Surfaces are playing more dominant a role as we enter the nanotechnology era. And, processing of surface is often kinetics limited. Various nanostructures are stabilized because of kinetic barriers, instead of thermodynamics equilibrium. In general, the kinetic barriers affect surface processing, at both nanoscale and larger scales. The thin film deposition process is one of those that are controlled by kinetics. Among many thin film structure characteristics, texture has been recognized as a key factor controlling performance. For example, the texture of aluminum interconnects in integrated circuits dictates their resistance to electromigration [1], and texture of TiN is preferred as mechanical coating and as barrier layers [2]. Aiming to control the texture during thin film deposition, we have developed an atomistic simulator (ADEPT), which is capable of simulating texture competition under realistic deposition rate at the atomic level [3,4,5]. The capability of ADEPT simulating textures at the atomic level depends on correct incorporation of atomistic processes, particularly diffusion processes. On a flat surface, whether an atom (or vacancy) diffuses by direct hopping or exchange, its diffusion can be relatively easily incorporated as far as the diffusion coefficient is known. Near edges of an island, the diffusion process becomes more involved. An adatom diffusing down an island may experience extra migration energy, the Ehrlich-Schwoebel (ES) barrier [6,7]. For a long time, attention on the ES barrier has been focused on island of monolayer thickness. In the following, we will describe a proposed kinetic barrier, its effects by Monte Carlo simulations, and the corresponding validation experiments. FACET-FACET BARRIER: A PROPOSAL From the ADEPT simulations [3] and transmission electron microscopy [8] – as shown in Figure 1, one can easily see large facets separated by a ridge on the surfac
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