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PREDICTIVE SCIENCE AND TECHNOLOGY IN MECHANICS AND MATERIALS

Yield and Deformation in Biaxially Stressed Multilayer Metallic Thin Films

[+] Author and Article Information
N. R. Overman, C. T. Overman, H. M. Zbib, D. F. Bahr

School of Mechanical and Materials Engineering, Washington State University, Pullman WA 99164-2920

J. Eng. Mater. Technol 131(4), 041203 (Aug 27, 2009) (6 pages) doi:10.1115/1.3183775 History: Received January 24, 2009; Revised March 18, 2009; Published August 27, 2009

Multilayer thin films of CuNb, CuNi, and CuNbNi with 20 nm individual layer thicknesses were fabricated by magnetron sputtering on oxide coated silicon wafers. The mechanical properties of the films were measured using bulge testing and nanoindentation. Elastic and plastic properties were determined for freestanding films in both square and rectangular window geometries. A finite element model of the window was used to determine the yielding behavior to account for stress concentrations in the membranes. The initial yield in a pressurized membrane, on the order of 400–500 MPa, is approximately one-half of the flow stress inferred from nanoindentation hardness results between 1.9 GPa and 3.4 GPa, and is a result of microscale yielding prior to uniform deformation in these high strength systems.

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Copyright © 2009 by American Society of Mechanical Engineers
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Figure 2

(a) Typical pressure-deflection curves showing elastic deformation in a freestanding CuNb square film; (b) an enhanced view of the highlighted region in (a) between 5 kPa and 10 kPa. Tests 1–3 show minimal deviation after repeated testing of the sample.

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Figure 3

Pressure-deflection curves of the “substrate” SiO2/Si support window without the multilayer film as well as the “film+substrate” for a CuNb film and substrate system. The presence of the film increases the stiffness equally in both the positive and negative pressure regimes compared with the substrate alone.

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Figure 4

Hardness measured using continuous stiffness indentation performed on the CuNb multilayer films with individual layer thickness of 20 nm. Each data set shown represents an averaged set of 7–10 indents measured from different areas of the wafer.

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Figure 5

Plastic deformation in freestanding CuNb films shown as (a) pressure-deflection and (b) converted to stress-strain. The specimen shown was loaded to a pressure of −67 kPa, where the film was allowed to deform at a constant nominal pressure. The film was then unloaded slightly and reloaded several times showing elastic unloading after repeated plastic deformation.

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Figure 6

FEA results showing the Von Mises stresses present in a square membrane with a side length of 4 mm

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Figure 1

Schematic illustrations of mounting geometry for bulge testing: (a) film with substrate backing before RIE as tested in Fig. 3. Orientations (b)–(d) depict freestanding films after removal of the SiO2 support window with (b) mounting configuration, leading to the delamination of the film, (c) showing a negative and (d) positive pressure orientation.

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