The influence of metallization on resonance frequency and temperature sensitivity of GHz operating III-Nitride SAW based sensor structures

Single SAW resonator structures with different configurations of the interdigitated transducer (IDT) were manufactured using e-beam lithography on AlN/Si and GaN/Si. These structures, resonating in the GHz frequency range, are targeted for wireless and battery-less operation sensing applications, in harsh environments. The influence of the IDTs' metallization thickness and type (Al, Au) on the resonance frequency was investigated; experimental results are compared with numerical simulations. Surprising results have been obtained for Au metallization on GaN/Si SAW resonators; the resonance frequency vs. metal thickness first increases to a maximum value. This behavior was explained considering the influence of the high acoustic impedance of Au on acoustic wave propagation localization. The variation of the resonance frequency vs. temperature has been also analyzed for different metallization thicknesses; it was observed that for a fixed finger/interdigit spacing width the sensitivity increases if the metallization layer thickness increases.