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Design of Rayleigh SAW Resonators for Applications as Gas Sensors in Highly Reactive Chemical Environments

This work presents the performance and important design considerations of the first successful highly efficient Rayleigh surface acoustic wave (RSAW) resonator filters using gold (Au) electrode structure. Au metallisation provides corrosion immunity when the devices are operated as polymer coated gas sensors in chemically reactive environments. It is shown that Au, due to its 7 times higher density than Al, provides about 13 times higher frequency sensitivity and a stronger velocity perturbation in resonant devices compared to their RSAW counterparts with aluminium (Al) electrodes. This results in strong excitation of unwanted surface skimming bulk wave (SSBW) modes and transverse wave guide modes (TWGMs) in such resonant Au devices. Practical guidelines for the choice of correct device geometry, Au thickness and spacer variations to maximize coupling to the desired RSAW mode, increase resonator efficiency and loaded Q, as well as minimize interference with parasitic TWGMs are given. Experimental 433 MHz two-port resonators and dual mode inline coupled filters with insertion loss in the -8 to -12 dB range and loaded Q values in the 4500 to 6000 range are presented with emphasis on their polymer coating and gas probing behavior