Ultrasonic Devices Employing Piezoelectric Ta_2O_3/Pyrex-Glass Structure

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This project aimed at the realization of composite diaphragm structure employing Ta_2O_5/Pyrex-glass structure for the application to high-performance ultrasonic wave devices operating in VHF to UHF ranges.Firstly, deposition of ZnO films by the use of target-facing type of sputtering method was investigated. The result showed that this sputtering method enables to deposit very dense and well-oriented films, which can be successfully applied to the ultrasonic transducers operating in UHF range. Note that the film quality is degraded due to the grow discharge caused by the oxidisation of the sputtering targets. It was shown that the substrate temoerature must be raised to 400ﾟC for the growth of piezoelectric Ta_2O_5. Since this temperature may cause fracture of the diaphragm, following experiments were carried out by using ZnO film instead of Ta_2O_5.Secondly, fabrication process for the piezoelectric ZnO/Pyrex-glass composite diaphragm structure was investigated. It was shown that the Pyrex-glass suffers much stress during the anisotropy etching, and the stress results in deterioration of device perfomances. For its suppression, the investigators proposed deposition of Si layr, compliance of which diffuses the stress throughout the diaphragm. In addition, it was also shown that strain in the diaphragm can be further reduced by the use of Si_3N_4 layr as a protection layr.Thirdly, the piezoelectric thin-film resonators was developed by using the diaphragm structure, and the resonant Q attained to 600 at the fundamental resonance frequency of 250MHz. Then the fabrication process was extended so as to float the diaphragm from the substrate, and the resonator operating in 500MHz was realized. This clearly exhibits possibility for the monolithic integration of high frequency ultrasonic devices with semi-conductor electronic circuitry.