Impact of SAW device passivation on RF performance

Passivation layers consisting of sputtered Al₂O₃ have been deposited onto SAW devices for the purpose of reducing the incidence of shorts. A coupling-of-modes model was used with one-port resonators, coupled resonator filters and test structures. The passivation layer stiffens the surface, with a velocity increase proportional to (t/λ), where t is the passivation layer thickness. Attenuation is increased slightly, producing a 0.25 dB increase in the loss of a one-port resonator at 314 MHz. The effect on reflectivity is minimal, and of much lesser importance to the designer.