Vibration characteristics of micromachined piezoelectric diaphragms with a standing beam subjected to airflow

This paper presents a novel configuration consisting of a micromachined diaphragm and a standing beam for sensing airflow. A piezoelectric Pb(Zr,Ti)O3 (PZT) film is deposited by spin-coating on the diaphragm to actuate and sense the vibration at its resonant frequency. A standing and extruded beam built on the diaphragm responds to the aerodynamic drag force, inducing mechanical strain, increasing the stiffness of the structure, and subsequently shifting the resonant frequency. The finite element method is used to demonstrate the first four mode shapes and predict the frequency shift of the proposed structure when subjected to airflow. The prototype microstructure is fabricated by combining the PZT sol–gel process and silicon micromachining. In particular, a two-step deep silicon etching process is developed to form the diaphragm structure with a standing silicon beam on it. Flip-chip bonding by solder balls is adopted to electrically connect the electrode pads to the circuit board. The characterization of the flip-chip bonded sensor chip exhibits superior piezoelectric and electrical properties. The wind tunnel test shows a 25 Hz resonant frequency shift for a wind speed of 10 m/s.