This paper presents three-dimensional (3D) models of high-frequency piezoelectric micromachined ultrasonic transducers (PMUTs) based on the finite element method (FEM). These models are verified with fabricated aluminum nitride (AlN)-based PMUT arrays. The 3D numerical model consists of a sandwiched piezoelectric structure, a silicon passive layer, and a silicon substrate with a cavity. Two types of parameters are simulated with periodic boundary conditions: (1) the resonant frequencies and mode shapes of PMUT, and (2) the electrical impedance and acoustic field of PMUT loaded with air and water. The resonant frequencies and mode shapes of an electrically connected PMUT array are obtained with a laser Doppler vibrometer (LDV). The first resonant frequency difference between 3D FEM simulation and the measurement for a 16-MHz PMUT is reasonably within 6%, which is just one-third of that between the analytical method and the measurement. The electrical impedance of the PMUT array measured in air and water is consistent with the simulation results. The 3D model is suitable for predicting electrical and acoustic performance and, thus, optimizing the structure of high-frequency PMUTs. It also has good potential to analyze the transmission and reception performances of a PMUT array for future compact ultrasonic systems.

In this paper, the converse magnetoelectric (CME) effect by electric field tuning of magnetization in an original heterostructure composed of a polycrystalline yttrium iron garnet (YIG) film and a lead magnesium niobate-lead zirconate titanate (PMN-PZT) ceramic is presented. The magnetic performances of the YIG films with different thicknesses under a DC electric field applied to the PMN-PZT ceramics and a bias magnetic field are investigated. All the magnetization-electric field curves are found to be in good agreement with the butterfly like strain curve of the PMN-PZT ceramic. Both the sharp deformation of about 2.5‰ of PMN-PZT and the easy magnetization switching of YIG are proposed to be the reasons for the strongest CME interaction in the composite at the small electric coercive field of PMN-PZT (4.1 kV/cm) and the small magnetic coercive field of YIG (20 Oe) where the magnetic susceptibility reaches its maximum value. A remarkable CME coefficient of 3.1 × 10⁻⁷ s/m is obtained in the system with a 600 nm-thick YIG film. This heterostructure combining multiferroics and partially magnetized ferrite concepts is able to operate under a small or even in the absence of an external bias magnetic field and is more compact and power efficient than the traditional magnetoelectric devices.

We report on the deposition of Ba 0.7 Sr 0.3 TiO 3 thin films, BST (70/30), by a sol-gel process. After deposition by spin coating, the film is annealed at 750°C, one hour in air. BST films have been deposited on (111) silicon substrates (bare and platinum coated) and on (0001) sapphire. X-Ray diffration patterns show that the films are polycristalline with perovskite structure and free of secondary phase. From SEM images, we infer that the film surface is smooth, the grain size is about 50 nm and the thickness of the BST layer is 300 nm. Electrical characterizations are mainly presented on BST deposited on platinized silicon. The dielectric constant l r is about 400 at 10 kHz and slightly decreases with frequency. The leakage current density is low: typically 10 nA/cm 2 at +5 Volts. At ambient temperature, the existence of an hysterisis cycle attest that a BST (70/30) film is in the ferroelectric state. As concerns the dissipation factor, the lower value has been obtained with a BST film deposited on sapphire: tg i = 0.014 at 100 kHz.

A complete study is given in this paper on the structural properties of Ba(Sr,Ti)O₃ (BST) thin films which present various preferred orientations: (111) and (001) fiber and epitaxial textures. The films are deposited in situ at 800 °C by sputtering on Si/SiO₂/TiO_x/Pt substrates and the orientation is controlled by monitoring the concentration of O₂ in the reactive plasma or by prior deposition of a very thin TiO_x buffer layer between BST films and substrates. The epitaxial films are obtained on (001)-alpha-Al₂O₃ substrates covered with TiO_x buffer layers. In order to analyze finely the preferred orientations, the texture, the microstructural features, and the anisotropy-related quantities such as residual stresses in the films, the conventional Bragg−Brentano θ - 2θ x-ray diffraction diagrams is shown not to be sufficient. So, we systematically used x-ray combined analysis, a recently developed methodology which gives access to precise determination of the structure (cell parameters and space group) of the films, their orientation distributions (texture strengths and types) and mean crystallite sizes, their residual stresses. This fine structural analysis shows important modifications between the film qualities which induce differences in BST films electrical behavior, permittivity, loss tangent, and tunability.