| Summary: | 碩士 === 國立高雄應用科技大學 === 機械與精密工程研究所 === 103 === The purpose of this thesis is to improve ultrasound attenuation in the air by reducing acoustic frequency of the capacitive micromachined ultrasonic transducer (CMUT). The ultrasound attenuation decreases with lower natural resonant frequency of membrane. There are three critical factors affecting natural resonant frequency of membrane including diameter, thickness and vibration mode. A new polymer-based CMUT with a large membrane diameter of 300m is developed to achieve low attenuation in the air.
In this study, the center deflection and natural resonant frequency of ultrasonic transducers are analyzed using ANSYS software. To lower natural resonant frequency the membrane diameter should be increased. At the same time the membrane thickness should be increased to avoid large vibration amplitude which may cause air compression. The air compression will excite higher mode of vibration and generate higher natural resonant frequency. Another method to avoid air compression is to add plate-holes on the membrane. However, the plate-holes are only added at the edge of the membrane in current design so the air compression effect still exists due to low hole ratio.
The new CMUT design has a membrane diameter of 300m, thickness of 10m and natural frequency of 0.378MHz. The CMUTs are successfully fabricated using film lamination process. The experiment test verifies the natural frequency is 0.37MHz which is less than half of our earlier design. The new CMUT can effectively reduce the natural resonance frequency and lower energy loss in the air.
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