论文和技术资料

In our work, simulations of a piezoelectrical micromachined ultrasonic transducer compatible with CMOS-MEMS process are presented. The piezoelectric material of the PMUT is AlN sandwiched between two Al layers and a top Si₃N₄ passive layer. The piezoelectric and the metal materials have cross-shaped slit sealed by the passive layer, allowing reliable operation in liquids. Below the bottom electrode, there is a cavity that allows transverse oscillations of the membrane. The device has squared design with a side of 40 um, adapted to resonate at frequencies in the 10 MHz range in liquid environment.

COMSOL Multiphysics^® software has been used with the aim of comparison between square-shaped PMUT with a slit and square-shaped PMUT without a slit.

To this end, Acoustics and MEMS modules were used and a Multiphysics coupling between Pressure Acoustics, Solid Mechanics and Electrostatics was set. The result is an investigation of different possibilities of the cross-slit, the top-electrode and the cavity dimensions which optimises the acoustic performance in Fluorinert and Water.

The dimensions of the PMUT are parametrized which allows for investigations of wide variation of devices. For example, one can vary the thicknesses of the materials, the size of the top electrode and the cavity, also, the length, and the width of the cross-slit.

The Stationary, the Eigenfrequency and the Frequency Domain studies are set as separated studies. This way one can double-check the outcomes of one study before using them as input of a subsequent study. Additional .mph file is dedicated for the Time Domain study which allows quick transition from frequency to time domains without readjustment of the settings which reduces the possibility of mistake along the workflow.

The device dimensions' ratio defined by its length along z-axis divided by the length along x- or y-axis is less than 1/10. Thus, a Swept Mesh is used in all structural domains which also enhances the computation time. The Acoustics domain is semi-spherical with open spherical boundary conditions, where tetrahedral mesh is applied.

As the acoustic pressure is related to the magnitude and the frequency of the membrane's oscillations, the product of these two values is used when different designs are compared. Our simulations show that without top-electrode optimization, the cross-slit of squared PMUT (40 um side) allows approximately ten times better performance in liquids (water, Fluorinert) in comparison with the same PMUT without cross-slit. If the top-electrode is optimized, the simulated enhancement due to the cross-slit is between two and two and a half times.

CMOS compatible MEMS PMUTs are in the process of substituting the current ultrasound transducers for sensing fluid speed and direction, for mixing and exciting particles (sonication), taking images (ultrasonography), non-destructive testing and many other purposes in various fields. Therefore, their efficiency is an important problem for the science and technology. Our study can be applied to wide variety of membrane-based PMUT devices providing better performance.