Intrinsic mechanical quality factor Q m of thin films evaluated by the GHz ultrasonic pulse-echo technique

A method for evaluating the intrinsic mechanical quality factor Q m of thin films is proposed. In this method, a film specimen is fabricated directly on the backside of the substrate. A piezoelectric transducer on the opposite side of the substrate is used for evaluation. The round-trip attenuation α in the film specimen is estimated by analyzing the difference between two echoes: one reflected from the substrate and the other from the film specimen. Q m is calculated from the estimated attenuation α using the relation Q m = πf/αV a, where V a is the acoustic velocity and f is the frequency. This GHz pulse-echo technique enables evaluation of Q m for a wide range of solid films, including piezoelectric, non-piezoelectric, metallic, amorphous, and other films. The precision of this method was confirmed through theoretical simulations based on Mason’s model. In this study, the Q m of several piezoelectric and metal films was estimated. The intrinsic Q m of polycrystalline ScAlN was found to decrease with increasing Sc concentration. In addition, Q m tends to increase with improved crystal orientation. In particular, a significant improvement in Q m from 1300 to 3600 was observed in epitaxial ScAlN thin films compared with the polycrystalline ones.