Comparison of Electrical and Microtensile Evaluations of Mechanical Properties of an Aluminum Film
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RELIABILITY prediction for thin films used in microelectronic interconnects and microelectromechanical system (MEMS) components has been a challenge for scientists and engineers for the past several decades. A significant amount of research has been aimed at understanding material behavior at small scales, in order to enable better reliability prediction for design purposes. Early work indicated that materials parameters such as yield strength, an important input for the reliable design of components exposed to strains, may not be extrapolated from bulk materials.[1,2] Later work confirmed that as film thickness approached the size scale of the microstructural features of the constituent film, certain mechanical properties such as hardness and strength increased for thinner films, while the elastic properties remained unchanged.[3,4] It is clear that the interaction between length scales associated with the thickness and microstructure of a film and the length scales associated with dislocation plasticity plays a major role in determining the strength of a particular film.[4,5] N. BARBOSA, III, Materials Research Engineer, R.R. KELLER, Group Leader, D.T. READ, Physicist, and R.H. GEISS, Materials Research Engineer, are with the National Institute of Standards and Technology, Boulder, CO, USA. Contact e-mail: barbosa@boulder. nist.gov R.P. VINCI, Assistant Professor, is with the Department of Materials Science and Engineering, Lehigh University, Bethlehem, PA 18015, USA. This article is based on a presentation given in the symposium entitled ‘‘Deformation and Fracture from Nano to Macro: A Symposium Honoring W.W. Gerberich’s 70th Birthday,’’ which occurred during the TMS Annual Meeting, March 12–16, 2006 in San Antonio, Texas and was sponsored by the Mechanical Behavior of Materials and Nanomechanical Behavior Committees of TMS. Article published online January 5, 2007.
2160—VOLUME 38A, SEPTEMBER 2007
Methods used for measuring mechanical properties in bulk materials are typically not suitable for use at micrometer and nanometer length scales due to difficulty in specimen preparation, specimen handling, and the application and measurement of loads and displacements. Uniaxial monotonic testing has long been acknowledged as the primary method for measuring tensile and cyclic properties of bulk materials. However, this approach is particularly difficult at the micrometer and nanometer scales. Initial attempts to perform uniaxial tensile tests on films (i.e., microtensile tests) were performed through careful modification of existing large scale test equipment and methods to grip and test film specimens tens to hundreds of micrometers thick.[6,7,8] As film dimensions decreased, new methods for improved specimen handling and test actuation were required. Specimens were made more robust by fabricating them on silicon substrates. In some cases, the Si supports were removed after specimen gripping,[9,10] and in other cases, the Si remained in place during actuation.[11] Handling and actuation were further improved through increasingly co
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