Laser diagnostics of mechanical and elastic properties of silicon and carbon films

A novel method has been developed for the determination of mechanical and elastic properties of thin films such as film thickness, density, Young's modulus and Poisson's ratio. In this technique short laser pulses (ns-ps) are used co excite a broad-band surface acoustic wave pulse, and a cw laser (Michelson interferometer, probe beam deflection) or piezoelectric foil detector is employed for time-resolved detection of the resulting surface displacements. In a hydrogen-terminated ideal silicon crystal the surface wave pulse shows no dispersion effect. However, a thin native oxide layer, normally present on the surface, leads to a linear decrease of the phase velocity with frequency. Partially this dispersion effect may be due to damage caused by lapping. A quantitative analysis of the shape of the surface wave pulse as a function of energy of the exciting laser pulse yields the threshold fluences for the melting and ablation of silicon. Doping of silicon leads to nonlinear dispersion, which was used to characterize the doping profile and elastic properties of the doped region. For amorphous hydrogenated silicon films, used in photovoltaics, the density and elastic constants were measured. Different carbon films with widely varying mechanical and elastic properties were studied. For thin fullerite films (C-60, C-70) the density and elastic constants were determined for the first time, showing that this is the softest form of carbon. The quality of amorphous diamondlike and polycrystalline diamond films was investigated by comparing the density and elastic constants with those of single-crystal diamond. Due to its high information content the method allows a reliable characterization of these films with a thickness in the micrometer range.