Bulge Test on Free Standing Gold Thin Films
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Bulge Test on Free Standing Gold Thin Films Yawen Li and Michael J. Cima Department of Materials Science and Engineering, MIT, Cambridge, MA 02139 ABSTRACT Bulge test was performed on gold and gold/nitride composite films with in plane sizes ranging from 20 to 200µm. The films were prepared using the microfabrication process of a drug delivery MEMS device, and the bulge test setup was constructed using some of the packaging components of the device. Incremental pressure at 5psi interval was applied to the films, whose deflection was measured using interferometry. The extracted gold film properties from bulge test on either pure gold films or composite films yielded comparable values. The lower modulus (126~168 GPa) comparing with the bulk gold could be due to the less dense microstructure of the evaporated gold film.
INTRODUCTION Thin films are widely used in the microelectronic and microelectromechanical systems (MEMS) devices. Although most of them are selected for their electronic, magnetic, or optical properties, their mechanical properties are equally important for the structural integrity and reliability of the devices. The bulge test is a well established technique for studying the mechanical properties of free standing thin films. In the bulge test, uniform pressure is applied from one side of a free standing thin-film "window", causing it to deflect to the other side (see Figure 1). Measurement of the deflection and pressure can be correlated to the stress and strain in the film to extract the film properties, as described by the generalized bulge equation [1]:
σ 0t
M t 3 (1) h a4 a Where P is pressure, σ0 is the residual stress, t and a are film thickness and radius, h is the center E , and E is the deflection, ν is the Poisson ratio, M is the biaxial elastic modulus, M = 1 −ν Young’s modulus, c1 and c2 are constants dependent on the film geometry. For square films [1], c1 = 3.393, c2 = 1.996 − 0.613ν (2) P = c1
2
h + c2
Figure 1. Schematic showing principle of bulge test
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Composite films have also been bulge tested. Rule of mixture is generally used to analyze the experimental data [2]: (σ 0t )composite = ∑σ 0i ti (Mt )composite = ∑ Ei ti (3) i i 1 −ν i Where i designates the ith layer of the composite structure. We report in this paper the bulge test on a series of gold and gold/nitride films. One unique feature of our bulge test is that the films are prepared using the microfabrication process of a drug delivery MEMS device so that some of the packaging components of the device could be readily used to construct the bulge test setup. The films have in-plane sizes in the range of 20 ~ 200 µm, much smaller than films used in conventional bulge tests (generally greater than 1mm). A direct consequence of this shrinking dimension is that the bending stiffness of the thin films becomes important in the film deformation, especially for films with in-plane sizes less than 100 µm. We therefore used a revised equation proposed by Vlassak to analyze our test [3]: σ t Et 3 Mt h + c2 4
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