Characterization of thin diamond-like carbon films by ultrasonic surface waves

The method of ultrasonic surface waves is suitable to determine the elastic modulus of thin diamond-like carbon films deposited on silicon wafers (111) by the laser-induced vacuum arc deposition (Laser-Arc) if frequencies in the range up to 100 MHz are used. Surface wave pulses have been generated in the thermoelastic regime by an eximer-laser (XeCl/308nm). The ultrasonic signals influenced by the film structure have been detected with a piezoelectric transducer. The surface wave phase velocity depending on frequency has been measured with a Fourier transformation technique. To calculate the elastic modulus of the films the inverse problem of surface wave dispersion in coated materials has been solved. Carbon films with a thickness of 300 nm deposited at different substrate temperatures in the range between 60 degree C and 500 degree C are studied. Young's modulus values up to 230 GPa were obtained for substrate temperatures smaller then 150 degree C. The very small modulus for T small er then 150 degree C suggest a drastic decrease of sp3/sp2 bonding ratio in the amorphous DLC-structure. These results correlate to optical studies. High frequency surface waves have been demonstrated to be a sensitive indicator for evaluating the C-film structure depending on the deposition conditions