A micromachined radio frequency bandpass filter comprised by thin film piezoelectric aluminum nitride (AlN) lamb wave resonators for wireless communication channel selection has been developed, fabricated and tested.
The AlN lamb wave resonator has a multilayered structure with aluminum interdigital transducer (IDT) on top and bottom of a c-oriented aluminum nitride thin film (Al/AlN/Al). The top and bottom IDTs can excite the lowest-order symmetric lamb mode in the AlN thin film plate and the resonant frequency is defined by the pitch of IDTs. The ladder type AlN lamb wave filters composed of electrically-connected series and shunt AlN lamb wave resonators; the acoustically-coupled AlN lamb wave filters utilizes multiple quarter-wavelength reflectors in between the resonators. The AlN lamb wave resonators shows a quality factor, Q over 2,000 in the ambient condition with sub-50Ω motional resistance at the frequency range from 300MHz to 900MHz. The ladder filter shows up to 1% fractional bandwidth and 6dB insertion loss with termination resistances of 500Ω. A 6-mask microfabrication process has been developed and characterized to create AlN lamb wave resonators and filters. The high quality AlN thin film is deposited by AMS RF sputtering tool under 400 °C and the top and bottom IDTs are defined through Chlorine RIE etching. The final structure is released by xenon difluoride gas etching in to the silicon substrate.
May 31, 2008
Lai, Y. (2008). Piezoelectric Aluminum Nitride RF Front-end Design for Wireless Communication. United States: University of California, Berkeley.