Low loss ladder-type IF SAW filter, used Au-Ge-Ni electrode

Design methodology, COM (Coupling-of-mode) model simulation and experiment results for a low loss 3-stage ladder type IF SAW filter are presented. To enhance electrode mass loading, Au-Ge-Ni alloy (density is 5 times that of aluminum) is used instead of traditional Al electrode. 36/spl deg/ Y-X LiTaO/sub 3/ is used as substrate material. Different electrode thickness (4.5% to 11.2% of a wave length with Al electrode) is simulated to estimate the frequency responses and then filters were fabricated on 36/spl deg/ LT for comparing the results. When the electrode thickness is 2000/spl Aring/ (about 0.9% of a wave length with Au-Ge-Ni electrode, it´s corresponding to 4.5% of a wave length with Al electrode), the center frequency is around 204MHz, the insertion loss is near 2.7dB, out-of-band rejection is around 15dB, and there is a large pitch in the center of pass-band. When electrode thickness increases, pass-band ripple and bandwidth becomes smaller, center frequency moves from 204 MHz toward 197MHz, and insertion loss increases. When the electrode thickness is 4800/spl Aring/(about 2.24% of a wave length with Au-Ge-Ni electrode, it´s corresponding to 11.2% of a wave length with Al electrode), the center frequency is around 197MHz, the insertion loss is near 4.3dB, out-of-band rejection is around 17dB, and the center of pass-band is flat. The same results can be gotten with the simulated COM program. When the thickness increases, pass-band becomes smooth. Filter response is better when the electrode thickness is greater than 7% for Al electrode, but it is only greater than 1.4% for Au-Ge-Ni electrode. So, for ladder-type IF SAW filter, it is better to use Au-Ge-Ni electrode instead of Al because lift-off process could be done easily. But when the electrode is too heavy, filter shape will be damped because of mass loading.