Laser diagnostics of C60 and C70 films by broadband 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, Youngʹs modulus, and Poissonʹs 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 (C60 and C70) with thicknesses of 0.7–2.1 μm 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 γmax = 0.32 for the C60 films and γmax = 0.18 for the C70 films. The 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 C60 films a density of 1.67 ± 0.02 g/cm3, Youngʹs modulus of 10 ± 2 GPa, and Poissonʹs ratio of 0.25 ± 0.08 were found. For the C70 films the corresponding values were 1.64 ± 0.02 g/cm3, 4 ± 1 GPa, and 0.35 ± 0.1.