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Plane-strain Bulge Test for Thin Films

Published online by Cambridge University Press:  03 March 2011

Y. Xiang ,
X. Chen  and
J.J. Vlassak
Y. Xiang
Affiliation:
Division of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138-2901
X. Chen
Affiliation:
Department of Civil Engineering and Engineering Mechanics, Columbia University, New York, New York 10027-6699
J.J. Vlassak*
Affiliation:
Division of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138-2901
*
a) Address all correspondence to this author. e-mail: vlassak@esag.deas.harvard.edu
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Abstract

The plane-strain bulge test is a powerful new technique for measuring the mechanical properties of thin films. In this technique, the stress–strain curve of a thin film is determined from the pressure-deflection behavior of a long rectangular membrane made of the film of interest. For a thin membrane in a state of plane strain, film stress and stain are distributed uniformly across the membrane width, and simple analytical formulae for stress and strain can be established. This makes the plane-strain bulge test ideal for studying the mechanical behavior of thin films in both the elastic and plastic regimes. Finite element analysis confirms that the plane-strain condition holds for rectangular membranes with aspect ratios greater than 4 and that the simple formulae are highly accurate for materials with strain-hardening exponents ranging from 0 to 0.5. The residual stress in the film mainly affects the elastic deflection of the membrane and changes the initial point of yield in the plane-strain stress–strain curve, but has little or no effect on further plastic deformation. The effect of the residual stress can be eliminated by converting the plane-strain curve into the equivalent uniaxial stress–strain relationship using effective stress and strain. As an example, the technique was applied to an electroplated Cu film. Si micromachining was used to fabricate freestanding Cu membranes. Typical experimental results for the Cu film are presented. The data analysis is in good agreement with finite element calculations.

Keywords

Mechanical propertiesThin film

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Articles
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Journal of Materials Research , Volume 20 , Issue 9 , September 2005 , pp. 2360 - 2370
Copyright
Copyright © Materials Research Society 2005

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