| The research object in this thesis is piezoelectric micromachined ultrasonic transducer (pMUT) for mobile phones and other portable mobible multimedia devices due to its directional sound emitting which is expected to sovlve both the noise pollution and personal privacy issue when using micro-loudspeakers in these devices in public space.In this thesis, the design considerations for pMUT are showed and followed with both transducer element and array optimization. The fabricating process and performance tests are given too. For the element optimazation, mechanics models are set up to analyze the relationship between the membrane amplitude and electromechanical coupling coefficient by discussing tope electrode size and the thickness ratio of PZT/Si layer. The obtained optimizing structure is analized by ANSYS to verify that whether it is suitble for the working frequence and bandwidth requirement. By establishing a linear array of point-source model, the array of discrete line is optimized, which is derived from the array of normalized directivity function, so as to bring in non-dimensional factor of the main lobe width, and then discuss both the element number and spacing to balance the sound directivity and emitting efficiency.This thesis also details PZT/Si bonding technology based on MEMS fabrication using the aforementioned optimizaing structure parameters. Three key technologies were elaborated further: the precise bonding for PZT bulk ceramics and SOI substrate, the thinning of PZT ceramics and the etching of PZT.The experimental results show that pMUT obtains good microstructure and ferroelectric properties, but frequency response and acoustic properties are not ideal. 30dB audible sound pressure is obtained at 20cm distance and the micro-speaker has weak directivity at 53.6kHz carrier frequency. The thesis also discusses the factor for the not-so-good acoustic properties of pMUT: fabricating process, circuit-channel design, transducer package and test methods. |
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