Discrete and continuous deformation during nanoindentation of thin films

Abstract

This paper describes nanoindentation experiments on thin films of polycrystalline Al of known texture and different thicknesses, and of single crystal Al of different crystallographic orientations. Both single-crystalline and polycrystalline films, 400–1000 nm in thickness, are found to exhibit multiple bursts of indenter penetration displacement, h, at approximately constant indentation loads, P. Recent results from the nanoindentation studies of Suresh et al. (Suresh, S., Nieh T.-G. and Choi, B.W., Scripta mater., 1999, 41, 951) along with new microscopy observations of thin films of polycrystalline Cu on Si substrates are also examined in an attempt to extract some general trends on the discrete and continuous deformation processes. The onset of the first displacement burst, which is essentially independent of film thickness, appears to occur when the computed maximum shear stress at the indenter tip approaches the theoretical shear strength of the metal films for all the cases examined. It is reasoned that these displacement bursts are triggered by the nucleation of dislocations in the thin films. A simple model to estimate the size of the prismatic dislocation loops is presented along with observations of deformation using transmission electron microscopy and atomic force microscopy. It is demonstrated that the response of the nanoindented film is composed of purely elastic behavior with intermittent microplasticity. The overall plastic response of the metal films, as determined from nanoindentation, is shown to scale with film thickness, in qualitative agreement with the trends seen in wafer curvature or X-ray diffraction measurements.