CHARACTERIZATION OF THE ELASTIC PROPERTIES OF AMORPHOUS-SILICON CARBIDE THIN-FILMS BY ACOUSTIC MICROSCOPY

A 590 MHz high frequency acoustic microscope has been used as a non-de structive local probe to measure the elastic properties of amorphous s ilicon carbide thin films supported as coatings on silicon substrates and as free-standing membranes. The films were amorphous hydrogenated and hydrogen free silicon carbide a-Si0.5C0.5(H) thin layers deposited by using two deposition techniques, namely plasma enhanced chemical v apor deposition (PECVD) and laser ablation deposition (LAD), Surface a coustic images showed a good homogeneity of the elastic properties of the films. Experimental Lamb mode velocities were deduced from the fas t Fourier transform (FFT) treatment of the local acoustic response, V( z), and compared with values obtained from calculated models taking in to account the different propagation modes of acoustic waves. The valu es of the longitudinal, V-L, and shear, V-T, velocities were determine d to be, respectively, 9462 and 5853 m/s for a-Si0.5C0.5:H PECVD films and 10340 and 6397 m/s for a-Si0.5C0.5 LAD films. Elastic parameters such as Young modulus, E, and Poisson ratio nu were deduced from these velocity values. In good agreement with the results obtained with oth er techniques (bulge and indentation measurements), the Young modulus was shown to increase with the Si-C bond density of the a-SiC films wi th values of 196 GPa and 273 GPa for PECVD and LAD films, respectively . The Poisson ratio was found to be independent of the microstructure of the SIC films with a constant value of 0.19.