Prof. Wu Tao’s research group at SIST is committed to developing Micro Electro Mechanical Systems (MEMS) devices using advanced sensing materials and micro/nano technology. Their research interests mainly include the coupling mechanism and sensing application of multiferroic materials, piezoelectric acoustic micro/nano devices and IC interface circuits, and magnetoelectric sensors. Recently, the group published four papers on RF filters, acoustic transducer PMUT and magnetoelectric sensor based on AlN/AlScN piezoelectric film in the journal Nature Microsystems & Nanoengineering and the 34th international conference on Micro Electro Mechanical Systems (IEEE MEMS2021). These achievements combine sensor design with micro/nano manufacturing, taking advantage of the small size and high performance characteristics of MEMS devices, and offering good prospects for promoting the application of device miniaturization. The following are the four achievements.
New AlN MEMS filter topology
5G communication allocates a wider frequency band in pursuit of a faster communication rate. The RF band-pass filter is used to select the desired signal and surpass the irrelevant signal, which is the key to communication at the front-end. MEMS filter stands out for its compact size, low cost and excellent filtering performance. In order to improve the bandwidth of MEMS filter, an AlN MEMS filter topology with extremely high bandwidth expansibility was proposed. The MEMS filter has been extensively explored in three directions: 1. the use of resonant modes with high coupling vibration modes; 2. the exploration of materials with high piezoelectric coefficients (such as scandium-doped aluminum nitride AlScN and lithium niobate LiNbO3); 3. the use of hybrid topology to expand the bandwidth of acoustic resonators.
In order to solve the problem of small bandwidth of piezoelectric acoustic resonant filters based on AlN, Wu’s group and their collaborators verified the integration of AlN Lamb wave resonator and lumped element. In this situation, with the use of only one resonator, the resonator with a coupling coefficient of 0.94 % can be extended to 5.6 % proportional bandwidth, which is about 12 times that of the traditional topology. This technology is expected to be used as a solution for high broadband communications such as bands N77/N79 of 5G. Prof. Wu Tao is the corresponding author.
Fig. 1: Diagram of the AlN resonator and new filter topology
Novel Lorentz force magnetic sensor
Based on vertical magnetic field detection of the transverse high-order bulk acoustic resonator (LOBAR), a novel Lorentz force magnetic sensor was designed. The sensor uses the Lorentz force generated by the vertical magnetic field to stimulate the resonance of the AlN film, converts it into an electrical signal through the piezoelectric effect, and establishes the relationship between the magnetic field intensity and the voltage amplitude. No magnetic materials involved in this design, which reduces the cost and avoids the hysteresis and saturation effect caused by them. Due to the use of high-order mode, the magnetic sensor can work at a high resonant frequency of 261 MHz under atmospheric pressure, and obtain a high bandwidth of 250 KHz. By reducing the size of the sensor, the device can work at a higher frequency of GHz. First year doctoral student Shao Shuai is the first author and corresponding author.
Novel AlN inductively coupled plasma etching technology
Wu’s group proposed a mechanism of AlN inductively coupled plasma etching, and they obtained an excellent etching result by comprehensive analysis and optimization of multi-parameters, with the angle of the etching sidewall the best result reported so far. Inductively coupled plasma etching is widely used in the fabrication of micro/nano devices based on AlN and AlScN films, and the etching result significantly affects the performance of the devices. Compared with the traditional studies, the optimization method of this research is more systematic and effective. Second year graduate student Luo Zhifang is the first author and corresponding author. ShanghaiTech University is the first unit to complete this work.
New method of ultrasonic distance measurement
The research group has studied a new method of ultrasonic distance measurement of ultrasonic transducer based on AlN film. In their design, the excitation signal of PMUT is superimposed on the output signal through circuit coupling, and then the relationship between the distance to be measured and the wavelength of the sound wave is determined by the phase information of the two signals. This method eliminates the echo interference of the traditional TOF ranging method and can improve the accuracy in short distance ranging. First year graduate student Cai Junxiang is the first author, Prof. Wu Tao is the corresponding author.
Fig. 2: Highlights of three papers published in the IEEE MEMS2021
Paper titles and links:
1. AlN MEMS Filters with Extremely High Bandwidth Widening Capability
paper link: https://doi.org/10.1038/s41378-020-00183-5
2.Wide Bandwidth Lorenz-Force Magnetometer Based on Lateral Overtone Bulk Acoustic Resonator
3. Optimization of AlN and AlScN Film ICP Etching
4. Increasing Ranging Accuracy of Aluminum Nitride PMUTs by Circuit Coupling