Seminar Series - 2001-2002

Characterization of thin-film elastic properties using surface acoustic waves

Dr. D. C. Hurley
Materials Reliability Division
National Institute of Standards & Technology
Boulder, Colorado

Date: Tuesday, February 5, 2002
Time: 2:30 p.m.
Place: W183 Nebraska Hall


We will describe nondestructive techniques to quantitatively characterize the elastic properties of thin films (thickness approximately 2 mm or less). Our technique is based on the optical generation and detection of surface acoustic waves, or SAWs, over a broad frequency range (10-400 MHz).  The surface waves are generated using a line-focused, pulsed laser and detected by a broadband Michelson interferometer. The frequency dependence of the phase velocity, or dispersion relation, is determined by measuring the wave displacement as a function of propagation distance. Results are analyzed using a new inversion algorithm based on the elastodynamic Green's function for wave propagation in an anisotropic layered system. To illustrate our techniques, we will discuss experiments on several thin-film specimens and interpret them using our analysis methods. Results for an aluminum film on a fused silica substrate and a molybdenum film on a single-crystal silicon substrate show the validity of our methods and are in good agreement with literature values. The same SAW methods were also applied to a series of titanium nitride films on single-crystal silicon wafers developed as wear-resistant coatings. The films' elastic properties were determined using both isotropic and anisotropic analysis approaches.  The elastic moduli were found to vary with film thickness and residual stress. Values determined for the film thickness were in very good agreement with destructive thickness measurements using scanning electron microscopy.


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