Characterisation of free-standing polycrystalline CVD diamond films by SAW-based laser ultrasonics

Knowledge of the mechanical and elastic properties of polycrystalline diamond films is necessary if one is to use these materials for mechanical, acoustic and sensing purposes. At present there is very little information available about these properties for films greater than a few microns thickness. In this work we inspect free-standing polycrystalline diamond films of thickness in the 100s of microns range by means of wideband surface acoustic Rayleigh and Lamb waves with frequencies up to 300 MHz, generated using thermoelastic excitation by a short-pulse duration laser and detectied with a PVDF foil steel-wedge transducer. Measured Rayleigh wave velocities provide useful data about the properties of black diamond, white diamond and highly oriented diamond, as well as demonstrating a sensitivity to film quality. These results represent a considerable step forward in acoustic characterisation of CVD diamond, in terms of accurate and useful acoustic and elastic measurements and as a nondestructive testing tool of film quality.