Development of Ultrasonic Image Sensor for Micomachine

本研究旨在开发一种复合膜片结构的二维超声换能器阵列，用于微型机械的图像传感器。首先，对结构的制作工艺进行了研究。采用“牺牲层”技术，在同一硅片上制造大量高密度的微型膜片。结果表明，在结构中加入极薄的硅层可以显著提高复合结构的成品率。这是由于在制造过程中产生的内应力通过Si薄膜的顺应性扩散到整个结构，从而大大降低了膜片的应变（弯曲）。此外，牺牲层的蚀刻被移至器件制造的最后一步。这种技术可以构造非常薄、面积很大的复合隔膜。利用这些技术，构建了工作频率超过77MHz的复合压电谐振器。其谐振Q值达到303，与采用大块各向异性刻蚀技术代替牺牲层技术所获得的值相当。作为下一步的工作，作者一直试图将这些制造工艺应用到超声波换能器的构造中。然而，由于膜片中剩余的应力，导致插入损失相当大的增加，因此没有取得良好的结果。通过这项工作，作者发现这种高频超声换能器辐射的超声波可以产生相当数量的液体流，并且效率很高。这是因为换能器需要实现非常高的声功率密度，这对于有效地引起非线性现象是必不可少的。实验结果表明，仅施加40mW的声功率，液体表面抬升5mm左右，与理论预测基本吻合。在此基础上，提出了采用高频超声换能器阵列对液体流动进行电气控制的超声微泵及其致动器。少

This research was aimed at developing the two-dimensional ultrasonic transducer array employing the composite diaphragm structure for the use of image sensors fo micro-machines. First, fabrication process for the structure were investigated. The "sacrificial layr" technique was employed so as to fabricate a large number of the miniature diaphragms with high density in the same wafer. It was shown that yield for the fabrication of composite structure can considerably be improved by adding very thin Si layr to the structure. This is due to the fact that the internal stress caused during the fabrication is diffused throughout the structure by the compliance of the Si film, which reduces the strain (bending) of the diaphragm dramatically. In addition, etching of sacrificial layr was moved to the final step of the device fabrication. This technique enables to construct very thin composite diaphragms with large area. By using these techniques, composite piezoelectric resonator operating in 7 … More 77MHz was constructed. Its resonant Q attained to 303, comparable to the value obtained by using the bulky anisotropic etching technique instead of the sacrificial layr technique.As a next step of the work, the authors has been trying to apply these fabrication process to the construction of the ultrasonic transducer. However, good results have not been achieved due to remaining stress in the diaphragm, which causes considereble increase in the insertion loss.Through this work, the authors found that considerable amount of liquid stream can be generated by the ultrasonic wave radiated by this kind of high-frequency ultrasonic transducers with high efficiency. This is because very high acoustic power density is realized by the transducer and it is essential so as to cause non-linear phenomena effciently. It was experimentally shown that, by applying acoustic power of only 40mW,liquid surface was lifted about 5mm which mostly coincides with the theoretical prediction. Then the authors proposed ultrasonic micropump and actuators which employ the high frequency ultrasonic transducer arrays for the electrical control of liquid streaming. Less