Discrete and continuous deformation during nanoindentation of thin films

This paper describes nanoindentation experiments on thin films of polycryst alline Al of known texture and different thicknesses, and of single crystal Al of different crystallographic orientations. Both single-crystalline and polycrystalline films, 400-1000 nn in thickness, are found to exhibit mult iple bursts of indenter penetration displacement, h, at approximately const ant indentation loads, P. Recent results from the nanoindentation studies o f Suresh ct al. (Suresh, S., Nieh T.-G. and Choi, B.W., Scripta mater., 199 9, 41, 951) along with new microscopy observations of thin films of polycry stalline Cu on Si substrates are also examined in an attempt to extract som e 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 fil ms for all the cases examined. It is reasoned that these displacement burst s are triggered by the nucleation of dislocations in the thin films. A simp le model to estimate the size of the prismatic dislocation loops is present ed along with observations of deformation using transmission electron micro scopy and atomic force microscopy. It is demonstrated that the response of the nanoindented film is composed of purely elastic behavior with intermitt ent microplasticity. The overall plastic response of the metal films, as de termined from nanoindentation, is shown to scale with film thickness, in qu alitative agreement with the trends seen in wafer curvature or X-ray diffra ction measurements. (C) 2000 Acta Metallurgica Inc. Published by Elsevier S cience Ltd. All rights reserved.