Nano-indentation of Copper Thin Films on Silicon Substrates

Indentation methods are finding increasing use in the study of mechanical properties of bulk and thin-film materials over a wide range of size scales (e.g., [1,2]). With the increasing sophistication of instrumented, depth-sensing indentation equipment and of computational methods to model the deformation of materials subjected to indentation, there is growing interest in studying the elastoplastic properties and residual stresses in thin films on substrates (e.g., [2,3]) using instrumented indentation. Nano-indentation studies of thin films provide the possibility of examining a variety of micromechanistic processes using different indentor tip geometries and size scales. Although uncertainties exist, there is a rapidly expanding theoretical and computational effort aimed at providing better interpretations of the elastic modulus, yield strength, strain hardening and internal stresses as well as deformation mechanisms in thin films by recourse to nano-indentation [1–3]. Furthermore, there is experimental evidence from indented bulk Fe-Si, Au and W single crystals that indentation methods could provide quantitative information about the nucleation of defects, which are ostensibly manifested as abrupt bursts in the penetration depth of the nano-indentor into the material [4–6]. Systematic studies of the mechanical response of thin films to indentation, however, have not been undertaken where the effects of film thickness, texture and grain size on the mechanisms of indentation-induced deformation, including defect nucleation processes, are examined. We present here experimental results on the nano-indentation of polycrystalline Cu thin films, of three different thicknesses but with approximately the same grain size and texture, on Si substrates. Care was taken to ensure that the indentation response was solely that of the film, with the deformation field of indentation confined fully within the film thickness, and that the results interpreted from such experiments are not influenced by the specific tip radius of the indentor. Copper was chosen as the material for the thin film in view of its recent emergence as the interconnect material of choice in microelectronic devices. Pergamon Scripta Materialia, Vol. 41, No. 9, pp. 951–957, 1999 Elsevier Science Ltd Copyright © 1999 Acta Metallurgica Inc. Printed in the USA. All rights reserved. 1359-6462/99/$–see front matter PII S1359-6462(99)00245-6