WebCAST seminar on "Materials Surface Engineering by Simultaneous Action of Multiple External Forces" by Dimitrios Maroudas Professor of Chemical Engineering at the University of Massachusetts at Amherst DATE: September 20, 2007, 2-4 pm EST Dial-in from the comfort of your office to hear the presentation Deadline to Register: September 17, 2007 (details at http://www.castdiv.org/WebCAST.htm) Abstract: Understanding the response of materials surfaces to the simultaneous action of multiple external forces is required for the systematic generation and stabilization of certain surface features and patterns that play important roles in the tailoring of materials properties and function. In this presentation, we focus on the surface morphological stability and dynamics of stressed crystalline solids, which underlies various materials processing and reliability problems in numerous technological applications ranging from aerospace to microelectronics and nanotechnology. An example of such an important problem in microelectronics is the electromigration-driven dynamics of void surfaces in mechanically confined metallic films that are used as device interconnections in modern integrated circuits. Surfaces of stressed elastic solids have been shown to undergo morphological instabilities. For example, the competition between elastic strain energy and surface energy can cause the growth of perturbations from a planar surface morphology under certain conditions and trigger the so-called Asaro-Tiller or Grinfeld instability. It has been demonstrated experimentally and computationally that a planar surface of a stressed elastic solid can evolve rapidly into a cusped surface, with smooth tops and deep crack-like grooves by surface diffusion. However, the effects of the simultaneous action of an electric field on the morphological response of a conducting stressed solid surface have not been explored systematically. In this presentation, we explore surface morphological response to the simultaneous action of electric fields and mechanical stresses of crystalline solid conductors, such as Cu or Al, and of voids in thin films of such conductors. The analysis is based on a surface transport model that accounts for curvature-driven surface diffusion, surface electromigration, and stress-driven surface diffusion along with surface diffusional anisotropy. The computational predictions for the surface morphological evolution are based on self-consistent dynamical numerical simulations according to the fully nonlinear surface mass transport model, which is solved self-consistently with the electric field and stress field distributions on the solid (or the void) surface computed through a Galerkin boundary integral method. First, we report results of linear stability analysis for the morphological response of a planar solid surface to the combined action of an applied electric field and mechanical stress, assuming that the solid responds elastically to stress. We derive a dispersion relation, which describes the growth or decay rate of a perturbation from the planar surface morphology of the stressed solid under the simultaneous action of the electric field. We find that application of a sufficiently strong electric field can stabilize the surface of the stressed electrically conducting solid material that would be otherwise vulnerable to surface cracking under certain thermomechanical conditions; therefore, the electric current protects the material against cracking and inhibits its damage. Furthermore, we report the effects on the surface morphological stability of key material properties, such as the strength of surface diffusional anisotropy and the material's texture that is set by the surface crystallographic orientation. We find that the morphological response of face-centered cubic metal surfaces with <111> crystallographic orientation is easier to stabilize than that of surfaces with <100> or <110> crystallographic orientation. In addition to the linear stability analysis, we report computational results for the morphological evolution of a solid surface perturbed from an initially planar morphology under the simultaneous action of an electric field and mechanical stress. The numerical results confirm the main conclusions of the linear stability analysis. Our findings can be used toward development of systematic surface engineering strategies for improved materials reliability over a broad range of electromechanical conditions. Next, we examine the surface morphological response of voids in metallic thin films under the combined action of electric fields and mechanical stresses. Our analysis predicts that, in the absence of stress, increasing the electric field strength, or the void size, or the strength of the diffusional anisotropy past certain critical values leads to transitions from steady states to time-periodic states; the latter states are characterized by wave propagation on the surface of the void, which migrates along the film at a constant speed. The transition onset corresponds to a Hopf bifurcation that may be either supercritical or subcritical, depending on the symmetry of the surface diffusional anisotropy that is determined by the crystallographic orientation of the film plane. We focus on low-symmetry anisotropy and analyze the current driven void surface morphological response under the simultaneous application of tensile biaxial stress starting from conditions close to the Hopf point in the stress-free case. Propagation of stable surface waves on the void is observed again as the applied stress level increases beyond a critical value. Further increase of the applied stress level leads to a period-doubling bifurcation associated with more complex surface wave propagation. Such period-doubling bifurcations continue with increasing stress level, setting the system on a route to chaos. With further increase in the stress level, the system exits from the chaotic regime to a periodic window characterized by a complex time-periodic state with three periods. Further increase in stress drives the system to another chaotic regime, through a period-doubling bifurcation sequence, and ultimately to film failure beyond a certain maximum stress level. Detailed characterization of the complex shape evolution is performed over the range of stress levels examined and the nature of the resulting chaotic state (strange attractor) is discussed. These results are used to motivate surface engineering studies toward formation of desirable surface patterns in solid material systems of interest in electronics, optoelectronics, energy technologies, and various areas of nanotechnology. Biographical Sketch: Dr. Maroudas is currently Professor of Chemical Engineering at the University of Massachusetts at Amherst. He received his Diploma from the National Technical University of Athens in 1987 and PhD at the Massachusetts Institute of Technology in 1992, both in chemical engineering. After a postdoctoral research fellowship at IBM T.J. Watson Research Center in 1992-1994, he was a faculty member at the University of California at Santa Barbara before moving to his current position. His research has been in computational materials science and electronic materials. His honors and awards include the CAREER Award from the National Science Foundation and the Camille Dreyfus Teacher-Scholar Award.