||In this study, T-ladder type thin film bulk acoustic wave filters had been fabricated based on thin film bulk acoustic wave resonators. The titanium (Ti) seeding layer and platinum (Pt) for bottom electrode were deposited on silicon substrates by a dual-gun DC sputtering system. Field-emission scanning electron microscopy, atomic force microscopy and the four-point probe method showed that the Pt bottom electrode deposited on the Ti seeding layer exhibited favorable characteristics, such as a surface roughness of 0.69 nm and a sheet resistance of 2.27 Ω/□. The ZnO piezoelectric film was deposited using the two-step deposition method by RF magnetron sputtering. Field-emission scanning electron microscopy, atom force microscopy and X-ray diffraction revealed that ZnO piezoelectric film exhibited excellent characteristics, such as a the high preferred c-axis orientation and a rigidly precise surface structure with surface roughness of 7.37 nm. The wet etching process is adopted to fabricate cavity of device. The concentration of 30 wt% KOH and etching temperature of 100 ℃ had been indicated appropriate for etching processes. Finally, the top electrodes of the devices are varied to approach the performances of device applications. The results showed the highest coupling coefficient (kt2) of FBAR device can be obtained using platinum top electrode. The high coupling coefficient of FBAR device is appropriate for wide passband filter. |
The annealing processes had been used in order to improve the characteristics of piezoelectric films. The stress of ZnO film has been improved from -1.656 Gpa to 0.611 Gpa through the annealing process. At the annealing temperature of 400℃, the ZnO piezoelectric film exhibited excellent characteristics, such as a large grain size with smooth surface. The quality factor of FBAR device using ZnO film with 400℃ annealing was better than that without annealing.
The optimal conditions of fabrication processes are adopted to fabricate top electrode, bottom electrode and piezoelectric film. The T-ladder type FBAR band pass filter was constructed by FBAR resonators. The frequency response is measured using an HP8720 network analyzer and a CASCADE probe station. The 3-dB bandwidth, insertion loss and band rejection of the T-ladder type thin film bulk acoustic wave filter are 79MHz, -3.5 dB and 8.4dB at 2,379MHz, respectively.