Investigation of the correlation between mechanical quality factor Q m and temperature coefficient of acoustic velocity TCV in sputtered-deposited SiO2 thin films

The correlation between Q m, TCV, and FT-IR spectra of sputter-grown SiO2 films was investigated using GHz pulse-echo technique. Thin film ultrasonic transducers were fabricated on the sapphire substrate acting as delay line, and SiO2 films under test were grown on the opposite sides of the substrate. The SiO2 films were grown by RF magnetron sputtering under various conditions. In this structure, two echoes, from the bottom of the substrate and from the bottom of the SiO2 film, are observed in the time domain impulse response. The difference in insertion loss between the two echoes corresponds to the round-trip propagation loss in the SiO2 film. The attenuation constant α/f 2 was obtained by quadratic fitting, and Q m was calculated from the relationship of Q m=πf/αV. The bulk velocity of the SiO2 film was acquired from film thickness and the time delay between the two echoes. TCV of the SiO2 film was estimated from the change in velocity during thermal heating of 25–100 °C. This method allows simultaneous and co-located measurement of both Q m and TCV, enabling reliable correlation analysis. As a result, we observed an obvious positive correlation between Q m and TCV. Moreover, a correlation was observed between the IR spectra and both Q m and TCV, which suggests that variations in the Si–O–Si bond may be a common factor influencing both Q m and TCV.