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 onto silicon (111) wafers by laser-induced vacuum arc deposition (Laser-Arc) if frequencies in the range up to 100 MHz are used. Surface wave pulses were generated in the thermoelastic regime by an excimer-laser (XeCl, 308 nm). Ultrasonic signals influenced by the film structure were detected by a piezoelectric transducer. The surface wave phase velocity, which is dependent on frequency, was measured using a Fourier transformation technique. To calculate the elastic modulus of the films the inverse problem of surface wave dispersion in coated materials was solved. Carbon films with a thickness of 300 nm deposited at different substrate temperatures in the range between 60 and 500 degrees C were studied. Young's modulus values up to 230 GPa were obtained for substrate temperatures below 150 degrees C. The very small modulus for T> 150 degrees C suggests a drastic decrease in the sp(3)-to-sp(2) bonding ratio in the amorphous diamond-like carbon structure. These results correlate with optical studies. High frequency surface waves were demonstrated to be a sensitive indicator for evaluating the structure of the carbon film as dependent on the deposition conditions.