ELASTIC CHARACTERIZATION OF ANISOTROPIC THIN FILMS USING SAW DISPERSION.

NIST is developing theoretical and experimental techniques for non-destructive measurement of elastic characteristics of anisotropic thin films. Presently there is a strong industrial interest in elastic characterization of thin films because they have applications in electronic, photonic, and magnetic devices and are also used as wear and corrosion resistant coatings on ordinary materials. Material parameters of interest for elastic characterization of a film are the elastic constants of the film, its density, and thickness. In general, the films as well as the substrate in modern materials are elastically anisotropic. Current theoretical and experimental methods of non-destructive characterization are not convenient for anisotropic layered solids.

Experimentally, NIST measures the phase velocity of a surface acoustic wave (SAW) over a broad frequency range. NIST created a laser-ultrasonic apparatus for broadband SAWs. The system incorporates a 0.2 ns pulsed laser for SAW generation and a Michelson interferometer detector with bandwidth over 800 MHz. NIST used this apparatus to measure dispersion relations in several samples including titanium nitride films on single-crystal silicon and stainless steel samples. The thickness of these films ranged from about 250 nm to 3500 nm.

For the purpose of analysis of the dispersion data for SAWs, NIST developed a computationally efficient representation of the Green's function to model the elastodynamic characteristics of an anisotropic thin film on an anisotropic substrate. In this model, the Green's function is represented in terms of a delta function of space and time in slowness space. For anisotropic solids, this representation is computationally more convenient than the convention Fourier reciprocal space and frequency representation. Our model can account for surface texture of the film and imperfect interfacial bonding between the film and the substrate. The elastodynamic model is used to calculate the dispersion of surface acoustic waves in the film and also provides an efficient algorithm for the inverse problem of characterizing the film from measured values of the dispersion. NIST applied this method to calculate SAW velocities in various isotropic and anisotropic films including textured polycrystalline films of TiN on single-cr ystal Si and to estimate the elastic constants of the films. Excellent agreement is obtained between the theoretical and measured values of the SAW velocities in most cases.