Plastic response of the native oxide on Cr and Al thin films from in situ conductive nanoindentation
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Ryan C. Major and David Vodnick Hysitron, Inc., Minneapolis, Minnesota 55344
John H. Thomas, III Characterization Facility, University of Minnesota, Minneapolis, Minnesota 55455
Jeff Parker, Mike Manno, Chris Leighton, and William W. Gerberich Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455 (Received 21 October 2011; accepted 6 December 2011)
Thin native oxide layers can dominate the mechanical properties of metallic thin films. However, to date there has been little quantification of how such overlayers affect yield and fracture during indentation in constrained film systems. To gain insight into such processes, electrical contact resistance was measured in situ during nanoindentation on constrained thin films of epitaxial Cr and polycrystalline Al, both possessing a native oxide overlayer. Measurements during loading of the films show both increases and decreases in current, which can then be used to distinguish between various sources of plasticity. Ex situ measurements of the oxide thickness are used to provide a starting point for elasticity simulations of stress in both systems. The results show that dislocation nucleation in the metal film can be differentiated from oxide fracture during indentation. I. INTRODUCTION
The use of in situ microscopy and indentation techniques to probe the initial stages of elastic–plastic deformation is now four decades old.1–8 Theories regarding the “staircase” yield events, seen as rapid increases in displacement in load-controlled indentation, have been presented. In metallic systems, the loading curves obtained in such measurements have suggested nucleation of discrete dislocations in the metal, as well as fracture of the native oxide.2–8 Although nucleation of dislocations in the underlying metal and fracture of the oxide overlayer are physically quite different, the resulting indentation load– depth profile is similar. In this article, electrical contact resistance (ECR) measurements are taken simultaneously with depth-sensing nanoindentation to deconvolute these two processes. Conductive indentation has previously been used in situ to investigate phase transformations in Si9 and GaAs,10 for measuring dielectric breakdown,11 to estimate the contact area under nanoindenter tips,12 to evaluate wear of ionic thin films,13 to probe the fracture of an oxide layer and subsequent pull-off forces,14 and to investigate the presence of organic contamination layers.15 These ECR experiments have shown that it is possible to extract qualitatively useful
information regarding the deformation of the material under the indenter, above and beyond that of a simple load–depth profile. Additionally, such experiments have demonstrated that the theoretical and experimental contact areas of a spherical tip and flat surface could be correlated to, but not completely quantified with, the Maxwell conductance model.16,17 The Maxwell model relates contact area to changes in conductance caused by a constriction in the electronic transport in the
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