Measuring the elastic modulus and residual stress of freestanding thin films using nanoindentation techniques

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Warren C. Oliver Agilent Technologies, Nanotechnology Measurements Division, Research and Development, Oak Ridge, Tennessee 37830

Maarten P. de Boer MEMS Technology Department, Sandia National Labs, Albuquerque, New Mexico 87185-1084

George M. Pharr University of Tennessee, College of Engineering, Department of Materials Science and Engineering, Knoxville, Tennessee 37996-2200; and Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6132 (Received 1 April 2009; accepted 19 June 2009)

A new method is proposed to determine the elastic modulus and residual stress of freestanding thin films based on nanoindentation techniques. 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 analysis is 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 mm wide, 0.55 mm thick, and 150, 300, and 500 mm long. The estimated modulus for the four freestanding films match the value measured by electrostatic techniques to within 2%, and the residual stress to within 19.1%. The difference in residual stress can be completely accounted for by thermal expansion and a modest change in temperature of 3  C. 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 freestanding thin films using nanoindentation. I. INTRODUCTION

Developing novel techniques to characterize the elastic modulus and residual stress of thin films is an active area of research in the fields of microelectromechanical systems (MEMS) and materials science. In the case of MEMS, a successful product requires the integration of small scale mechanical and electrical components functioning together as sensors, actuators, and switches. As noted by Spearing, the MEMS industry has relied heavily on the past 50 years of research by the microelectronics industry in electrical characterization and fabrication techniques.1 This research is a key reason for the success of the microelectronics industry, as it forms the basis to quickly and reliably simulate complex devices and thus avoids the need to incorporate extensive prototyping. If the MEMS industry is to realize its full potential, similar design simulation packages will be required, and successa)

Address all correspondence to this author. e-mail: [email protected] or [email protected] DOI: 10.1557/JMR.2009.0360

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http://journals.cambridge.org

J. Mater. Res., Vol. 24, No. 9, Sep 2009 Downloaded: 14 Mar 2015

ful implementation of the simulation process will depend on the accuracy of the mechanical properties used as inputs. Properties such as strength and residual stress are expected to vary with the microstructure of the film and the fabrication tec

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