Research On Fabrication And DC-Bias Regulation Of AlN-Based Piezoelectric Micromachined Ultrasonic Transducer

Owing to the excellent directionality and penetration,ultrasound can provide real-time high-resolution anatomical and Doppler imaging,which is widely used in the field such as material science,acoustics,biomedical engineering,and geophysics.As microelectromechanical devices that can emits and receives ultrasound waves,ultrasound transducer is a key component in ultrasound imaging systems and has been the focus of research in the academic community.Piezoelectric micromachined ultrasonic transducer(PMUT)exhibits the potential to promising applications due to the high sensitivity and spatial resolution.As a lead-free environment-friendly piezoelectric material,Aluminum Nitride(Al N)has been gradually used in PMUT due to its compatibility with complementary metal oxide semiconductor technology.However,restricted by the low piezoelectric constant and accompanying low emission sensitivity,it is vital to optimize the structure design of PMUT to achieve high electromechanical coupling coefficient.Moreover,the bandwidth of PMUT is narrow compared to other ultrasonic transducers,which greatly increase the complexity of the PMUT fabrication by coupling structures.Hence,it is urgent to develop new methods of PMUT bandwidth regulation.To meet the requirements of ultrasonic monitoring and ultrasonic imaging systems on the scale of ultrasonic transducer array,the key technologies of the preparation of high-density ultrasonic transducer array is explored in this paper,which could be used to solve the technical problems of traditional ultrasonic transducer limited by narrow bandwidth and difficult array.At the same time,DC bias regulation is conducted in this paper.In this paper,the theoretical basis of PMUT,multi-physical field simulation and parameter optimization of Al N-based PMUT are studied.It is revealed that the best performance is achieved when the upper electrode diameter is 70% of the film diameter.The final diameter size of PMUT device is 360 μm,the thickness of piezoelectric layer is 1 μm,and the thickness of vibration layer is 5 μm.Al N-based PMUT devices were fabricated by Micro-ElectroMechanical System(MEMS)manufacturing process,and wet etching with better material selection was used to complete the graphic chemical sequence of Al N in this process.The mechanical and electrical properties of the PMUT devices show that the resonant frequency of the PMUT device was 505.94 k Hz and the electromechanical coupling coefficient was 1.06%,which were assessed using a Doppler vibrometer and impedance analyzer,respectively.The experimental results of the influence of DC bias on PMUT performance show a variation in resonant frequency in the range of 4.08 k Hz.The resonant frequency,bandwidth,and quality factor of the device exhibit a linear variation under a DC bias voltage ranging from-30 to 30 V.The reason is that DC bias causes the film to produce controllable stress and changes the support structure of the device and affects the anchor loss.Finally,the bandwidth of PMUT was increased by 60.2% by stacking several response curves,which is calculated from the resonant frequency variation of PMUT under DC bias.