An Experimental Study of the Work Hardening Behavior of Metallic Thin Films

Harvard University, Cambridge MA

Investigators

Abstract

TECHNICAL SUMMARY: The objective of this project is to develop experiments and models to elucidate the mechanical behavior of materials in small, constrained volumes with a particular emphasis on how a material work-hardens and on the dislocation processes that take place in the vicinity of a free surface or a hard passivating layer. Many micro-electronic and micro-mechanical devices consist of dissimilar materials put together in complex architectures. Components of these devices are often subjected to very high stresses ? stresses so high, that they can often not be supported by the same material in bulk form. Insight in the mechanical behavior of materials at these small length scales and accurate models to describe this behavior are essential to the design and fabrication of reliable devices. This objective will be achieved through a combination of micro-mechanical experiments and discrete dislocation simulations. The experiments will focus on measuring the stress-strain behavior of polycrystalline freestanding thin films under a range of passivation conditions. Recovery mechanisms will be investigated using creep and relaxation experiments. The dislocation substructure that develops in the vicinity of a free surface or a passivating layer will be investigated using grazing incidence X-ray scattering at the Stanford Synchrotron Radiation Laboratory and by means transmission electron microscopy at Harvard. The stress-strain behavior of polycrystalline films will be modeled using discrete dislocation simulations that will incorporate appropriate recovery mechanisms to model the hardening behavior of the films. NON-TECHNICAL SUMMARY: This award will lead to a better understanding of the deformation behavior of thin films and coatings, making it possible to design and fabricate more reliable devices at the micro- and nano-scales. Industries that stand to gain the most are the MEMS and semiconductor industries. The nascent field of low-cost, large-area, flexible electronics will also benefit from a fundamental understanding of the mechanical behavior of the thin metallic films they use as electrical interconnects. Results of the investigation will be published in archival journals and will be posted on the archival web site www.iMechanica.org hosted by the Harvard School of Engineering and Applied Sciences. iMechanica serves as a discussion forum for young scientists with a goal of enhancing communication among researchers and of making mechanics and materials more accessible to a wider public. The project will also contribute to both graduate and undergraduate education. The award will support one graduate student and will provide research opportunities to several undergraduate students. The Principal Investigator has long engaged undergraduate students of diverse background in his research projects and this award will ensure that this practice continues. Research under this award will contribute to the development of a graduate course on thin-film mechanics. The project has an international component through collaboration with the Technical University of Delft in the Netherlands. They will contribute expertise in discrete dislocation modeling and bring this capability to Harvard University.