NIST MEASURES THE ELASTIC PROPERTIES OF THIN FILMS WITH ULTRASOUND.

Surface coatings are used throughout industry to give ordinary materials extraordinary resistance to wear and corrosion as well as to provide specialized electrical or magnetic properties for device applications. Knowledge of the elastic properties of the coating is important not only for estimating the residual thermal stresses but also for ensuring that the desired microstructure is present. During the past 2 years, an ultrasonic technique has been developed in which lasers are used to launch and detect surface waves that propagate along the coated surface of a thick substrate. Because the transducers use optical interactions with the substrate and the film, they are non-contacting in nature and can operate at frequencies extending up to hundreds of megahertz. This allows the surface wave velocity to be measured with high accuracy at wavelengths extending down to tens of microns. Theoretical models that relate the elastic constants of the film and substrate to the surface wave velocity are available in the seismology and surface acoustic wave device literature but they are cumbersome for deducing the film properties from measurements of the surface wave velocity.

NIST has advanced the technology both experimentally and theoretically by dealing with elastically anisotropic materials at frequencies approaching 500 MHz and by developing a Greens function formalism that speeds up and simplifies the deduction of elastic properties from surface wave velocity measurements. These new capabilities recently have been used in a cooperative program with an Australian laboratory to measure the elastic properties of a series of submicron thick TiN films deposited on silicon single crystal wafers. These films were of particular interest because they were elastically anisotropic and supported a range of residual stresses.