Abstract :
The main objective of this research is to enhance the sensitivity of ultrasonic detector device to be applied in the object detection and image acquisition of longer distance. In this experiment, MEMs process technology and sacrificial layer technology are used to prepare micro-bridge structure so as to reinforce the ultrasonic signal sensitivity of PZT thin film device. In this research, Al is adopted as the sacrificial layer of this device, and the material of the structural layer is SiO2. Through the selective etching between structural layer and sacrificial layer, the sacrificial layer can be removed, and micro-bridge structure PZT ultrasonic sensor device can be obtained. Meanwhile, in order to further reinforce the sensitivity of the sensor device, in this research, the prepared micro-bridge structure ultrasonic sensor devices are connected in multiple devices way, and 32 elements are connected into a sensor unit. Then several sensor units are connected in parallel, and the characteristic test is conducted at the same time too. Then 32×1, 32×2, 32×4, 32×8, 32×16, 32×31 parallel-connected PZT piezoelectric thin film micro-bridge structure ultrasonic sensor devices are formed respectively. For the micro-bridge structure ultrasonic sensor device prepared by this research, small experimental water tank is used as test environment, then transmitting array device and microbridge structure ultrasonic sensor device are fixed at the same side with a distance of 55cm to the target object, then through an amplifier, the signal is amplified by 60dB, the oscilloscope is used for the measurement. The echo signals of 32×1, 32×2, 32×4, 32×8, 32×16, 32×31 parallel-connected micro-bridge structure ultrasonic sensor devices are measured respectively to compare the situation of the increase of the sensitivity from the multiple sensor devices. From the experimental results, - t can be seen that 32X31 micro-bridge structure ultrasonic sensor device is the device of the best echo signal. From the experimental results, it can be seen that for the ultrasonic sensor devices with more sensor devices connected in parallel, the detectivity of the device can be enhanced. For the best 32X31 ultrasonic sensor device obtained in this experiment, we have performed characteristic measurement on such device. For the 32X31 micro-bridge structure ultrasonic sensor device prepared in this research, its resonance frequency is 4.39MHz, and the anti-resonance frequency is 25.79MHz. The remanent polarization is 7.02 uC/cm2, coercive electric field is 7.6 kV/cm. In the underwater test, better sensitivity is found at 4~5KHz, and the sensitivity at 5KHz~1MHz is worse, however, when the test frequency is close to high frequency of 1M~10MHz that is close to resonance frequency, the sensor device will have larger voltage output, hence, better sensitivity can be obtained, the best sensitivity is near -173 dBreV/uPa. At 600KHz, the sensitivity is -205.99 dBreV/uPa. For the sound field part, at low frequency, high frequency and 600KHz, the sound field figure will be close to a circle, that is, there is no specific direction.
