Research On The Performance Of MEMS Ultrasonic Transducer Based On AlN Thin Film

As a critical component in ultrasound applications,the performance of ultrasonic transducers determines the performance of ultrasonic application systems to a large extent.High-frequency piezoelectric micromachined ultrasonic transducer(PMUT)are widely used in medical imaging,medical diagnosis,industrial diagnosis,non-destructive testing,etc.However,high-frequency ultrasonic transducers are usually relatively small in size and require high performance,which puts forward higher requirements for PMUT device design,material selection,and processing methods.In this paper,aluminum nitride(AlN)piezoelectric film is selected as the functional material,and a three-dimensional model of high-frequency PMUT is established.The PMUT diaphragm structure with the best performance under a specific structural size is designed by optimizing the mode of the diaphragm.The design process is developed,and the device is fabricated and subjected to characterization and testing.It mainly includes the following aspects:(1)According to the working principle of PMUT,the three-dimensional structure model of PMUT is designed and established.An air-coupled backplane PMUT is proposed.The threedimensional model consists of a substrate,a silicon device layer,and a molybdenum-aluminum nitride-molybdenum(Mo-AlN-Mo)sandwich piezoelectric structure layer.By simplifying the model to an edge-fixed diaphragm,the key performance indicators such as vibration equation,displacement,resonance frequency,sensitivity,and emission voltage response of PMUT were analyzed and calculated.(2)The PMUT vibration mode is optimized,and a circular diaphragm,a square diaphragm,and a hexagonal diaphragm are designed.The parametric scanning analysis of the key dimensions of the PMUT shows that under the same size,the fundamental frequency of the PMUT with the hexagonal diaphragm is higher than that of the circular and square diaphragms.Compared with the classic circular diaphragm,the receive sensitivity of the square diaphragm PMUT is increased by 8.5%,and the sensitivity of the hexagonal diaphragm PMUT is increased by 10.7%.The maximum emission sound pressure level of the hexagonal diaphragm PMUT is6.6 times that of the circular diaphragm.(3)The structural parameters are determined according to the finite element simulation results,and then the process flow and mask are designed,and the process is addressed.The fabricated devices were characterized by laser confocal microscopy and scanning electron microscopy.The resonance frequencies of the circular diaphragm PMUT and the hexagonal diaphragm PMUT are measured by an impedance analyzer to be 10.44 MHz and 11.46 MHz,respectively.The calculated electromechanical coupling coefficients are 2.3% and 2.1%,respectively.(4)A test platform is set up to test the transmission and reception performance of PMUTs.It is measured that both circular and hexagonal PMUTs had good linearity in their transmission and reception.The hexagonal PMUT has a better transmission response than the circular PMUT,and the transmission response decreases gradually with increasing distance.The sensitivity of the PMUTs is tested,and within the range of 5-13 MHz,the sensitivity of PMUT with the hexagonal diaphragm is better than 15 μV/Pa,up to 29 μV/Pa,and the sensitivity of PMUT with the circular diaphragm is higher than 2 μV/Pa.