LASER DIAGNOSTICS OF MECHANICAL AND ELASTIC PROPERTIES OF SILICON ANDCARBON-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 pul ses (ns-ps) are used co excite a broad-band surface acoustic wave puls e, and a cw laser (Michelson interferometer, probe beam deflection) or piezoelectric foil detector is employed for time-resolved detection o f the resulting surface displacements. In a hydrogen-terminated ideal silicon crystal the surface wave pulse shows no dispersion effect. How ever, a thin native oxide layer, normally present on the surface, lead s to a linear decrease of the phase velocity with frequency. Partially this dispersion effect may be due to damage caused by lapping. A quan titative analysis of the shape of the surface wave pulse as a function of energy of the exciting laser pulse yields the threshold fluences f or the melting and ablation of silicon. Doping of silicon leads to non linear dispersion, which was used to characterize the doping profile a nd elastic properties of the doped region. For amorphous hydrogenated silicon films, used in photovoltaics, the density and elastic constant s 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 investigat ed by comparing the density and elastic constants with those of single -crystal diamond. Due to its high information content the method allow s a reliable characterization of these films with a thickness in the m icrometer range.