Dislocation Dynamic Simulations of Metal Nanoimprinting
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Dislocation Dynamic Simulations of Metal Nanoimprinting
Yunhe Zhang1 , Erik Van der Giessen2 and Lucia Nicola1 1 Department of Materials Science and Engineering, Delft University of Technology, 2628 CD Delft, The Netherlands 2 Zernike Institute for Advanced Materials, University of Groningen, 9747 AG Groningen, The Netherlands
ABSTRACT Simulations of metal nanoimprinting by a rigid template are performed with the aim of finding the optimal conditions to retain imprints in a thin film on a substrate. Specifically, attention is focussed on the interface conditions between film and substrate and on the template shape. Deeper imprints are obtained when the interface between film and substrate is penetrable to dislocation motion. When the protruding contacts of the rigid template are closely spaced the interaction between neighboring contacts gives rise to material pile-ups between imprints. INTRODUCTION Metal nanoimprinting is of great technological interest due to its potential applications in miniaturized systems. While the most common technique to achieve nanoimprints in metal is lithography, e.g. [1], imprinting by mechanical indentation of the film has recently been suggested as a promising alternative approach, see e.g. [2]. The objective of this study is to investigate numerically the ability of a metal film on substrate to retain imprints when indented by a rectangular wave pattern. We focus our attention on the nature of the interface between film and substrate, and on the effect of the spacing between protruding contacts. In this respect the size dependence of plastic properties at the sub-micron size scale is expected to cause a non-trivial interaction of the plastic zones underneath the contacts [3]. At the length scale of interest for miniaturized devices, conventional finite element simulations based on classical continuum plasticity fail in predicting localized stresses and deformations. The approach used in this study is 2D discrete dislocation plasticity [4], where plasticity in the metal film is described in terms of the collective motion of discrete dislocations. The discreteness of dislocations, with an evolving density, is the key element for size dependent plasticity, giving rise to a large deviation of submicron-structure behavior from that of bulk metal. In addition, large number of dislocations gliding out the metal free surface leave surface steps that are comparable in size to the depth of the final imprint. Dislocations are modeled as line singularities in an otherwise isotropic linear elastic medium. Constitutive rules are supplied for the glide of dislocations as well as their generation, annihilation and pinning at point obstacles. The simulations track the evolution of the dislocation structure during loading, unloading and relaxation and provides an accurate description of the final imprinted profile.
Figure 1. Two dimensional model of a metal thin film on substrate imprinted by a rigid template with rectangular wave profile. Each unit cell contains three flat contacts
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