A method for evaluating intrinsic mechanical quality factor <i>Q</i>m of thin solid films such as ScAlN, GaN, metal materials using GHz pulse-echo technique

Abstract

A method for evaluating the intrinsic mechanical quality factor Qm of thin films with the GHz pulse-echo technique is proposed. In this method, a film specimen is deposited 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: reflected from the substrate and the film specimen. The Qm is then obtained using the estimated attenuation α from the relationship of Qm = πf/αVa, where Va is the acoustic velocity and f is the frequency. This method is capable of evaluating the Qm of a wide range of solid films, including piezoelectric films, metal films, amorphous films, and others. In this study, the Qm of ScAlN, AlN, ZnO, GaN, and several metal films was estimated. The intrinsic Qm of polycrystalline ScxAl1−xN decreased from 3800 to 1300 at 2 GHz when Sc concentration increased in the range of x of 0–0.4. In addition, Qm tends to increase with improved crystal orientation, and in particular, a significant improvement in Qm from 1300 to 3600 was observed in epitaxial ScAlN thin films compared with the polycrystalline ones.