Measuring the elastic modulus and the residual stress of free-standing thin films using nanoindentation techniques
E.G. Herbert, W.C. Oliver, M.P. de Boer, A. Lumsdaine, G.M. Pharr
Agilent Technologies, Inc., US
Keywords: thin film, elastic modulus, residual stress
Abstract:A new method is proposed to determine the elastic modulus and residual stress of free-standing thin films based on nanoindentation techniques that reduce the effect of thermal drift in the load. The experimentally measured stiffness-displacement response is applied to a simple membrane model that assumes the film deformation is dominated by stretching as opposed to bending. Dimensional and finite element analysis (FEA) are used to identify appropriate limitations of the proposed model. Experimental verification of the method is demonstrated for Al/0.5 wt% Cu films nominally 22 µm wide, 0.55 µm thick, and 150, 300, and 500 µm long. The estimated modulus for the four free-standing films match the value measured by electrostatic techniques within 2%, and the residual stress within 19.1%. The difference in residual stress can be completely accounted for by thermal expansion and a modest change in temperature of 3 degrees C. A desire to examine the impact of potential anomalies in the experimental results, such as misalignment of the tip of the nanoindentation head with the surface of the bridge or adhesion of the tip with the surface of the bridge, motivates an in-situ examination of the experiment within a scanning electron microscope (SEM). Numerous experimental pitfalls are identified and discussed. Collectively, these data and the technique used to generate them should help future investigators make more accurate and precise measurements of the mechanical properties of free-standing thin films using nanoindentation techniques.