A novel pulse-echo method for direct measurement of the temperature coefficient of velocity (TCV) in thin solid films

As thermal stability of SAW and BAW filters has become critically important, temperature coefficient of the bulk acoustic wave velocity (TCV) values of various film material are required. We propose a novel method to directly evaluate intrinsic TCV of a film material. The proposed pulse-echo technique utilizes a structure consisting of a substrate with an ultrasonic transducer on its front surface and the target film on its backside. TCV is determined from the temperature dependence of the time delay between two echoes: one reflected from the bottom of the substrate and the other reflected after a round-trip propagation within the film. This method enables measurement of TCV of piezoelectric films, non-piezoelectric films, and metal films, even when the TCV of the substrate is unknown. We assessed the validity of this method through both theoretical simulations and experiments. Furthermore, TCVs of various films were measured, including metal films suchas Pt, Au, and Mo; piezoelectric films such as ZnO and ScAlN. For Sc x Al1-x N films, a clear trend of decrease of TCV with increasing Sc concentration was observed, which shows agreement with previously reported results for free standing resonator.