Impact of Transducer Aspect Ratio on the performance of AlN Hybrid SAW/BAW Resonators

SummaryIn this study, we investigate the impact of the aspect ratio of transducers on the electromechanical coupling factor ${\mathbf{k}}_{\mathbf{t}}^2$ of hybrid SAW/BAW (HSB) resonators. Finite element calculations of the harmonic admittance suggest that the optimum ${\mathbf{k}}_{\mathbf{t}}^2$ for the two modes of interest of these devices, are attained for aspect ratios of approximately 2.4 for the pseudo-SAW mode and 3.4 for the pseudo-BAW mode. HSB resonators with aspect ratios ranging between 0.6 and 8 are fabricated to investigate this dependence experimentally. The dependence of the electromechanical coupling factor with aspect ratio matches the theoretical trend. It however also proves the need to consider a partial etching of the piezoelectric layer. For the pseudo-SAW mode, we find an experimental maximum ${\mathbf{k}}_{\mathbf{t}}^2$ of 1.5% at h
AlN
/a=3.6 close to the theoretical value when the piezoelectric layer is only etched over 60% of its thickness. For the pseudo-BAW mode, we find an experimental maximum ${\mathbf{k}}_{\mathbf{t}}^2$ of 1.7% at h
AlN
/a=3.6..