Film-thickness considerations in microcantilever-beam test in measuring mechanical properties of metal thin film

Since metal thin films have broad applications in micromechanical devices, their mechanical properties are very important in designing and controlling such devices. In this study, the Young's modulus and yield strength were estimated by using a microcantilever beam-bending technique. Al and Au cantilever beams of various thicknesses and lengths were fabricated by silicon bulk micromachining and were loaded in bending by a nanoindenter; displacement was simultaneously obtained at high resolution. The load-displacement data showed that the yield strengths of the two films increase with decreasing film-thickness. However, the Al and Au films showed different mechanical behaviors: the yield strength of the An film followed the grain-size dependency of Hall-Petch type, but that of the Al film was not fully described by the grain-size effect alone. This difference arises mainly from the different surface conditions of each film. The effect of the surface on a film's mechanical properties is discussed and a microstructure effect was suggested that a modified misfit dislocation theory used to predict the yield strength. (C) 2003 Elsevier Science B.V. All rights reserved.