Farid Takali

Ordinary Differential Equation applied to ultrasonic wave Propagation in piezoelectric material

The propagation of ultrasonic waves is still a topic of interest due to its applications in surface acoustic wave devices (SAW). Research works related to propagation in various media as multilayer, thin film and semi infinite media can be found in the recent literature. Applications of these studies include technological areas as electronic systems and sensors. Acoustic wave propagation in piezoelectric media is basically achieved by the use of the ordinary differential equations method ODE. Our purpose is to elucidate how ODE is carried out leading to a best identification of the involved electromechanical partial waves. The eigenvector equation related to ODE method has been solved in three cases depending on the symmetry of the sagittal plane. Therefore the fundamental acoustic tensor is reformulated with respect to the existing coupling between the elastic partial waves and the electrical one. The new formulation of ODE is useful during propagation studies either in elastic materials or piezoelectric ones. The obtained results are illustrated on the well known piezoelectric material ZnO, which is assumed to be hexagonal.

ZnO and AlN/diamond layered structure for SAW devices with subsurface damage

Diamond has the highest SAW velocity of around 11000 m/s among all materials. Therefore, diamond is of particular interest for use in the design of gigahertz-band SAW devices recently. To induce high-velocity SAW, diamond has to be combined with the piezoelectric film, such as aluminum nitride (AlN) or zinc oxide (ZnO) which has been deposited on diamond. Diamond surface acoustic wave (SAW) has been used to boost the frequency of thin film filters made of piezoelectric materials. An analytical study is carried out on the propagation of Rayleigh surface waves in a piezoelectric wafer with subsurface damage. The region of subsurface damage is considered to be a functionally graded piezoelectric thin film. Contrary to the analytical approach, the adopted numerical methods, including the ordinary differential equation (ODE) and the stiffness matrix method (SMM), treat the electrical and mechanical gradients. Results show that the first mode exhibits the largest coupling coefficient (5.74%) for the structure ZnO/diamond. For the mode 3 the K2 curve shows the optimal value (11.81%) for the structure AlN/diamond. Realizations of additional devices with optimum normalized thicknesses determined by calculation are in progress in order to confirm the high potential of the new ZnO/AlN/diamond structure for high performance and high frequency saw filters.