Design of Piezoelectric Micro-machined Ultrasonic Transducers for Wideband Ultrasonic Radiation in Air

Abstract

This paper explains the design method of new wideband pizeoelectric micro-machined ultrasonic transducer(pMUT) for radiation in air. One of the methods to obtain wide frequency band in single device is modeling the transducer to multi-resonance system. The new pMUT device proposed in this paper is modeled to multi-resonance system by adding acoustic structures at both frontside and backside of the pMUT membrane. The acoustic structure at the frontside of the membrane(so called radiating part) is modeled to the acoustic waveguide. The acoustic structure at the backside of the membrane(so called packaging part) is modeled to the acoustic closed cavity.

The membrane part, radiating part and packaging part is designed to have wide frequency band by fast analysis based on lumped parameter model of pMUT device. The acousitc amplification effect of radiating part is also considered in radiating part design. The membrane and the radiating part is designed to secure enough width of frequency band in acoustic volume velocity curve. The packaging part is designed to flatten the wide frequency band. The various pMUT device with various membrane thickness and center frequency can be designed by the wideband pMUT device design algorithm which is proposed in this paper. The algorithm designs pMUT device by lumped element method.

The single wideband pMUT device is finally designed by accurate analysis including the dimension of piezoelectric operation layer based on finite element model. Finally designed pMUT device in this paper has 86.7 ~ 118.0kHz (+-3dB) frequency band. The new pMUT device also has higher sound pressure level than conventional pMUT device(maximum 30.8dB more sound pressure level at frequency 112.6kHz). Also it has very high electro-acoustic efficiency (maximum 101.8%) calculated by numerical method.

The manufacturability of the newly designed pMUT device is validated by designing the fabrication flow of the device. The new pMUT device is fabricated on the separated wafer level. The membrane and radiating part is fabricated on the device wafer level and packaging part is fabricated on cap wafer level. The fabricated two wafers are bonded by eutectic bonding to single pMUT device.