HIGH Q METAL STRIP SSBW RESONATORS USING A SAW DESIGN

An experimental study of metal strip surface skimming bulk wave (SSBW) resonators using a surface acoustic wave (SAW) design is presented. Characteristics of SSBW and SAW resonators fabricated with the same photolithographic mask are compared and discussed. High Q low loss SSBW resonators are achieved using a conventional two-port SAW resonator design and taking special care of the distance L between both interdigital transducers (IDT), the metal thickness h/λ (λ — acoustic wavelength) and the finger-to-gap ratio. Best overall performance of the SSBW devices in this study is achieved at L = nλ/2 − λ/4, (compared with L = nλ/2 − λ/8 for SAW resonators), Á/λ = 1.6%, (compared with 2% for SAW) and finger-to-gap ratio close to 1. The best device fabricated shows an unloaded Q of 5820 and an insertion loss of 7.8 dB at 766 MHz. The SSBW resonant frequency shows a stronger dependence on the metal thickness than the SAW one. This problem, however, is readily solved by frequency trimming using a CF4 plasma etching technique. SSBW resonators can be trimmed by 0.2% down in frequency (compared with 0.05% for SAW) without affecting their performance. © 1990, IEEE