Micro-accelerometer with film bulk acoustic resonator on diaphragm

In view that the beam structure of FBAR (Film Bulk Acoustic Resonator) micro-accelerometer has such disadvantages as insufficient cantilever beam thickness and complex micro-fabrication process of embedded-FBAR structure, an FBAR-on-diaphragm structure of FBAR micro-accelerometer is presented. The elastic diaphragm is formed by SiO2/Si3N4 composite films, which is not only susceptible to IC compatible integrated processing of the silicon micro proof-mass and the FBAR, but also conducive to improving sensitivity and temperature stability of the FBAR micro-accelerometer. The preliminary performance analysis was made on the FBAR-on-diaphragm micro-accelerometer which integrates the SiO2/Si3N4 bi-layer composite diaphragm and the Si mass inertia force sensing structure with the aluminum nitride FBAR detecting components, and the feasibility of the FBAR-on-diaphragm structure is verified. Based on finite element modal analysis and static simulation, the natural frequencies of the FBAR-on-diaphragm structure and stress distribution of the diaphragm under the inertia loads are obtained. The calculated maximum stress is used as stress load to apply to the piezoelectric film in the FBAR, and combined with the calculated elastic coefficient-stress relation of the AlN with wurtzite structure according to the first principle. In this way, the maximum elastic coefficient variation of the aluminum nitride under variant inertia force can be roughly predicted. With the help of the RF simulation software ADS, by changing the longitudinal wave velocity corresponding to the elastic constants with variant inertia loads, and comparing the resulted resonant frequencies of the accelerometer under null and different inertial loads, its frequency shift and sensitivity can be characterized. Further analysis of the simulation results reveals that the first-order natural frequency of the SiO2/Si3N4 diaphragm is quite far away from the higher ones, which means less cross coupling. In addition, under the inertial force load, its resonance frequency will upshift with the sensitivity of several-kHz/g magnitude, and its acceleration-frequency shift characteristic curve is of good linearity. ©, 2015, Editorial Department of Journal of Chinese Inertial Technology. All right reserved.