Plasticity Size Effects In Freestanding Thin Films:

Abstract

Over the past decade, there has been a substantial thrust to reduce the size of electronic
and electromechanical systems to the micron and sub-micron scale by fabricating devices out of thin
film materials. Successful device development requires a thorough understanding of material
mechanical properties as a function of device characteristic dimension. At this scale, specimen
geometry and dimensions are similar in size to the material microstructural features. We present a
new on-chip membrane deflection experiment specially designed to investigate material elastic
behavior, plasticity (including size effects in the submicron regime), and fracture. The study examines
plasticity size effects in freestanding fcc thin films in the absence of macroscopic strain gradients.
Experimental results including transmission electron microscopy will be presented to demonstrate that
indeed strong plasticity size effects exist and to highlight their possible sources.
Current shortcomings of plasticity theories at the submicron scale motivated us to examine a
multiscale modeling approach to capture the aforementioned size effects. Our modeling is twofold.
Firstly, 3D Discrete Dislocation Dynamics mesoscopic simulations are carried out using the software
PARANOID+ to understand qualitatively dislocation-dislocation, dislocation-film surfaces, and
dislocation-grain boundary interactions in freestanding films. Secondly, a grain level FEM model
based on Crystal Plasticity is used to take into account initial grain orientations and texture in a
representative volume element (RVE) of the tested films. The geometry of the RVE is based on
polyhedrons obtained by Voronoi tessellations. A criterion based on discrete dislocation dynamics
results is incorporated into the crystal plasticity continuum model and used to describe the onset of
plasticity in each grain. The efficiency of this modeling strategy will be discussed.