||In this thesis, Rayleigh-mode and Sezawa-mode surface acoustic wave devices, and SMR-based (solidly mounted resonator, SMR) thin film bulk acoustic wave devices were employed to construct the UV sensors. The oscillators are composed of acoustic wave devices, high-frequency amplifier and matching networks. Due to the fact that the different acoustic wave devices are associated with the different propagating behaviors, electromechanical coefficient and resonance characteristics, they lead to the diversely sensing properties. Although Rayleigh-mode and Sezawa-mode SAW devices are both constructed by a ZnO sensing layer, they operate with different resonance behaviors and propagate with different phase velocities in the layered structures. Therefore, they result in different frequency shifts and sensitivities while illuminating UV light on the surface of ZnO thin films. As to the SMR device, the acoustic waves are confined within the ZnO piezoelectric layer sandwiched between two metal electrodes and then resonance as standing waves. In general, thin film bulk acoustic wave devices, which are SMR devices in this thesis, possess a higher operating frequency and better frequency response than those of SAW devices. Therefore, it is expected that UV sensors based on SMR devices will lead to an excellent performance.|
The Rayleigh-mode SAW-based UV sensors consisted of a 3μm-thickness ZnO thin film for sensing UV light and a 2mm-thickness LiNbO3 substrate for generating surface acoustic waves in the ZnO/ LiNbO3 layered structure. Because surface acoustic waves travel along the surface within the depth of one wavelength, 32 μm herein, most of them propagate in the LiNbO3 substrate. SAWs were perturbed slightly and consequently resulted in an unsatisfactorily maximum frequency shift of 63.75 kHz when a UV light intensity of 1250 μW/cm2 was illuminated on the surface of ZnO thin film. Because ZnO films in this thesis are used as the sensing layer for UV light, we adjusted the sputtering parameter of deposition temperature to improve their crystalline properties and further enhance the sensitivity of ZnO/LiNbO3 layered SAW devices. Finally, the maximum frequency shift was raised to 264 kHz with the same UV light intensity using the deposition temperature of 400 ℃.
The ZnO thin films in the ZnO/Si layered structure were simultaneously employed as the piezoelectric layer for generating SAWs and the sensing layer for UV light. Therefore, all of the acoustic waves propagate within the ZnO thin films and are easier disturbed than the devices operated with the ZnO/LiNbO3 layered structure. This accounts for the relatively large frequency shift of 1017 kHz with the UV light intensity of 551 μW/cm2.
The ½ λ type SMR device was adopted to construct the UV sensor due to their better resonance characteristics than those of ¼ λ type. As can be seen from the results that SMR-based UV sensor presented better UV sensing properties compared with SAW-based UV sensors. The reasons for the considerable frequency shifts and sensitivities can be attributed to that SMR-based sensor possesses a shorter resonance wavelength and a larger electromechanical coefficient than those of SAW-based devices. Finally, the maximum frequency shift of 552 kHz can be obtained when the illumination intensity of UV light was 212 μW/cm2.