A MEMS piezoelectric in-plane resonant accelerometer based on aluminum nitride with two-stage microleverage mechanism
Introduction
Accelerometers are widely used in applications from state-of-art military equipment to the most common consumer electronics. A wide variety of accelerometers has been designed and implemented based on a number of different techniques [1]. There are two major sensing categories based on principles to detect acceleration: displacement sensing and force sensing. Displacement sensing accelerometers transduce acceleration to displacement of movable masses, and the displacement can then be picked up by optical, capacitive, piezoresistive [2], piezoelectric [3] or tunneling principles. Accelerometers based on force sensing detect force applied on the proof masses to know acceleration [4]. As one of the force sensing methods, resonant accelerometers detect acceleration by measuring frequency shift of resonators. Resonant frequency sensing outputs a quasi-digital signal, which is easily demodulated by frequency counting techniques. Such sensing scheme also has an advantage of large dynamic range, which gives the resonant accelerometers more potential in various applications.
Electrostatic and piezoelectric transductions are frequently used in the resonant accelerometers for actuation and sensing. T. Roessig et al. demonstrated an electrostatic MEMS resonant accelerometer with two differential DETF resonators and a bias-place proof mass, which has sensitivity of 2.4 Hz/g [5]. Then they introduced leverage mechanisms to the resonant accelerometer and improved the sensitivity to be 45 Hz/g [6]. S. X. Su et al. made a comprehensive summary about the theory of microleverage mechanism and improved amplification factor to 80 and enhanced sensitivity to 158 Hz/g [7]. Claudia Comi further improved sensitivity of an electrostatic resonant accelerometer to 430 Hz/g by using a new centrosymmetric structure [8]. Compared to electrostatic resonant accelerometers which are limited by pull-in effect [9], resonant accelerometers based on piezoelectric transduction have more advantages in generating and detecting out-of-plane movement, which indicates that more desired modes of the resonators can be obtained in piezoelectric resonant accelerometers. This advantage gives the piezoelectric resonant accelerometers more potential and attractiveness. AlN is considered to be one of the most promising piezoelectric materials, for its post-CMOS compatible fabrication process as well as stable piezoelectric and mechanical performance under harsh environment. Roy H. Olsson et al. firstly realized a MEMS piezoelectric AlN resonant accelerometer by differential DETF resonators and a centre-place proof mass with sensitivity of 3.4 Hz/g under resonant frequency of 890 kHz [10]. Fabian T. Goericke improved the design of AlN resonant accelerometer by introducing microleverage mechanism and optimized the arrangements of each component [11]. Although the sensitivity was enhanced from 3.4 to 18.1 Hz/g under resonant frequency of 332.1 kHz, the value is still low comparing with those from the electrostatic resonant accelerometers.
In this paper, we introduce a new MEMS piezoelectric AlN in-plane resonant accelerometer with two improved designs: (1) each element is optimized to form a centrosymmetric structure so that the length of resonator in a given area is maximized; (2) a two-stage microleverage mechanism is used to amplify the inertial force from the proof mass, which effectively improves the sensitivity. With the above two improvements, the presented piezoelectric resonant accelerometer has sensitivity of 28.4 Hz/g and relative frequency change Δf/fn of 201 ppm/g, both of which are better than the previously reported data.
Section snippets
Working principle of the accelerometer
A typical resonant accelerometer consists of several resonators as sensing elements and a proof mass, as shown in Fig. 1. When an external acceleration is applied on the proof mass, tensile or compressive stress is exerted on the resonators and the stiffness of the resonant beam changes, which causes a shift of resonant frequency of the resonator. The frequency shift of the resonator is proportional to the applied acceleration, and is relatively small compared with the base frequency. The two
Design and fabrication of the device
Considering all these theories and trade-off between various parameters, the optimized structure dimensions are summarized and listed in Table. 1. The amplification factor of the two-stage microleverage is 108 according to the FEA results. The two DETF resonators are put at each side of the proof mass to form a centrosymmetric configuration, in which the length of the DETF resonators are maximized. Compared with the work of Olsson [10] and Gabriele [11], our design has higher space efficiency
Experimental results and discussion
For testing, the electrode pads of the device were connected to a printed circuit board (PCB) by wire bonding, as shown in Fig. 6(a). To measure frequency shifts of the resonators under different accelerations, a network analyzer (E5061 B Keysight) was used to read the frequency response of the device, as shown in Fig. 6(b). To verify frequency shifts under different accelerations, the PCB was installed on a rotary table which can use gravity to generate accelerations from 1g to −1g by adjusting
Conclusions
An AlN piezoelectric in-plane resonant accelerometer in size of 2000 × 1600 μm2 has been designed, fabricated and tested. Each element of the accelerometer is optimized through analytical analysis and FEA. A structure with centrosymmetrically-distributed DETF is proposed so that the length of resonator was maximized with high space efficiency. A two-stage microleverage mechanism is used as an amplifier of the inertial force from the proof mass. Our device has better performance than the previously
Acknowledgements
This work is supported by the “National Natural Science Foundation of China (51475423, 51275465)”, the “Zhejiang Provincial Natural Science Foundation of China (LY14E050018, LZ16E050001)”, the “Science Fund for Creative Research Groups of National Natural Science Foundation of China (51521064)”, and the “Zhejiang Open Foundation of the Most Important Subjects”.
Yixiang Wang received the B.Eng. degree from Zhejiang University, Hangzhou, China in 2014. Now he is pursuing master degree in Zhejiang University, Hangzhou, China. His research interest is microelectromechanical systems (MEMS) sensors and actuators.
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Yixiang Wang received the B.Eng. degree from Zhejiang University, Hangzhou, China in 2014. Now he is pursuing master degree in Zhejiang University, Hangzhou, China. His research interest is microelectromechanical systems (MEMS) sensors and actuators.
Hong Ding received the B.Eng. degree from Zhejiang University, Hangzhou, China, in 2013. Now he is pursuing PhD degree in Zhejiang University. His research interest includes MEMS, resonators and resonant accelerometers.
Xianhao Le received the B.Eng. degree from Zhejiang University of Technology, Hangzhou, China in 2014. Now he is pursuing PhD degree in Zhejiang University, Hangzhou, China. His research interest is microelectromechanical systems (MEMS) sensors and actuators.
Wen Wang received the B.Eng. degree from Hangzhou Dianzi University, Zhejiang, China, in 1990, the M.Eng. degree and the Ph.D. degree from Zhejiang University, Hangzhou, China, in 1992 and 1996, respectively. From 1996–2012, he worked in Zhejiang University. From 2009–2010, he worked in the Center for Precision Metrology of the University of North Carolina at Charlotte, NC, USA as a visiting researcher. In July 2012, he joined the School of Mechanical Engineering, Hangzhou Dianzi University, Zhejiang, China, as a professor. His research interests include Micro/Nano measuring and control technology, coordinate measuring machines, precision engineering and mechatronics.
Jin Xie received the B.Eng. degree from Tsinghua University, Beijing, China, in 2000, the M.Eng. degree from Zhejiang University, Hangzhou, China, in 2003, and the Ph.D. degree from Nanyang Technological University, Singapore, in 2008. From 2007–2011, he worked in Institute of Microelectronics, Singapore. In June 2011, he joined the Department of Mechanical Engineering, University of California, Berkeley, CA, USA, as a post-doc researcher. In October 2012, he joined the Department of Mechanical Engineering, Zhejiang University, Hangzhou, China, as a professor. His research interests include microelectromechanical systems (MEMS) design and processes, inertial sensors, resonators, acoustics and vibration measurement.