LASER DIAGNOSTICS OF C-60 AND C-70 FILMS BY BROAD-BAND SURFACE-ACOUSTIC-WAVE SPECTROSCOPY

A novel method is used 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 (nanosecond to picosecond) are used to excite a surface acoustic wave (SAW) pulse and laser probe beam deflection or a piezoelectric foil detector are employed for time-resolved detection of the resulting surface displacements. Fourier transformation of the oscillatory signals detected at distances of several millimeters to centimeters yields the dispersion of the phase velocity, which can be used for the accurate determination of film properties. Fullerite films (C-60 and C-70) with thicknesses of 0.7-2.1 mu m on silicon and quartz-glass substrates were investigated. The nonlinear dispersion curves were obtained experimentally with a maximum value of the product of film thickness and SAW wave number of gamma(max) = 0.32 for the C-60 films and gamma(max) = 0.18 for the C films. The 70 frequency bandwidth was limited by the attenuation of the surface acoustic waves in the fullerite films. The film parameters were evaluated by fitting the measured dispersion curve to a theoretical model. For the C-60 films a density of 1.67 +/- 0.02 g/cm(3), Young's modulus of 10 +/- 2 GPa, and Poisson's ratio of 0.25 +/- 0.08 were found. For the C-70 films the corresponding values were 1.64 +/- 0.02 g/cm(3), 4 +/- 1 GPa, and 0.35 +/- 0.1.