压电式微加工超声传感器的关键技术研究

Ultrasound plays an important role in almost all aspects of our lives. Although advances have been achieved by bulk ultrasonic transducers based on thickness vibration mode, its laborious manufacturing process limits the realization of cost-effectiveness, miniaturization, and 2D arrays for advanced ultrasonic imaging. With the development of microelectromechanical systems (MEMS) technology, MEMS-based piezoelectric micromachined ultrasonic transducers (PMUTs) have advantages in device miniaturization, cost-effective manufacturing with large volume, and advanced ultrasonic array design. Therefore, this thesis starts a systemic investigation on basic theories and key technologies for PMUT, including electromechanical–acoustic coupling theory, structural design and optimization methods, fabrication process, and its application in the contact stress sensing. These investigations will provide the theoretical basis and the technical platform for the development and applications of PMUT. The main contents of this thesis are as follows:

First, electromechanical and acoustic models for the PMUT are established. An equivalent circuit model of electromechanical-acoustic coupling is also established for PMUT cells with circular or annular diaphragms on the basis of the analogous relationship between mechanical and electrical parameters. The equivalent circuit model systems of fluid-coupled PMUT array are developed by introducing the mutual radiation and resonant cavity auxiliary structures to the coupling models of the PMUT cell. The proposed array equivalent circuit models become versatile and adaptable through modification and optimization. Compared with the finite element analysis method, the proposed equivalent circuit models can eliminate the influence of meshing quality on the calculation accuracy and considerably improve the calculation efficiency. Thus, the requirements of accurately predicting the performance of PMUT cell and array are addressed.

Then, crosstalk behaviors among the PMUT cells are studied on the basis of the established electromechanical–acoustic coupling equivalent circuit model. Moreover, crosstalk control and optimization methods are presented considering the modification of cell distribution, excitation method, and the adjustment of acoustic impedance distribution across the array surface. An annular and circular diaphragm coupled PMUT array and PMUT arrays integrated with the resonant cavities are presented on the basis of the proposed crosstalk control methods. The proposed new PMUT arrays not only verify the crosstalk control method but also improve the ultrasonic emission sensitivity.

Afterward, PMUT chips are designed on the basis of existing fabrication capacity and testing conditions. The chips are also fabricated based on low-temperature wafer-bonding technology. Then, testing circuits are designed and fabricated for the PMUT chips, which are packaged and tested with an impedance analyzer to obtain their frequency responses. Results indicate the excellent agreement between the model and the experimental findings, which only have a relative error of around 5%. A pulse-echo working condition of the chip is also tested considering the basic calibration of the PMUT chip.

Finally, a novel PMUT excitation method based on parametric amplification has been presented. And this excitation method is verified by theoretical model with lumped parameter method and experiment results based on bimorph piezoelectric circular plate. In this method, the parametric signal periodically modulates the bending stiffness of the circular plate and makes the modulated bending stiffness coupling with the bending vibration of the plate. By this means, the vibration amplitude of PMUT can be largely enhanced without complicating the structure and fabrication process.