Evaluation of acoustic properties of thin films using piezoelectric overtone thickness-mode resonators

Acoustic properties of thin films are very important parameters to design and fabricate thin film devices such as piezoelectric thin film resonators with acoustic quarter-wave multilayers. In this paper, a simple evaluation method for acoustic properties, such as density and elastic stiffness, of thin films is presented. In this method, the acoustic properties are evaluated from the measured resonance frequency change of high overtones of a piezoelectric thickness-mode resonator due to deposition of a test film on the resonator surface. The resonance frequencies of higher overtones change being influenced by stiffness as well as density. Thus, the density and stiffness of the film as the unknown parameters can be determined so as to minimize the sum of the square errors between measured and calculated resonance frequencies for many overtones. This method was applied to evaluation of acoustic properties of various thin films by using a Z-cut LiTaO₃ plate as the piezoelectric resonator. The resonance frequencies calculated using the evaluated density and stiffness agreed well with the measured values, suggesting that the properties of thin films were properly evaluated