Effect of geometry on the performance of MEMS aluminum nitride trampoline resonators in longitudinal resonance

The performance of a microscale aluminum nitride piezoelectric resonator in the shape of a trampoline is analyzed using three-dimensional finite element simulations. The air-suspended resonator is supported by beams and is designed to respond to longitudinal through-thickness vibrations. The device is targeted to operate at UHF frequencies (3 GHz) suitable for wireless filtering applications. Energy loss due to material damping is accounted for in the model. Other sources of damping are considered. We analyze if and how the material thickness, number of beams and beam length affect the resonator performance. This is intended to provide useful information at the design stages and eliminate the high costs associated with manufacturing a filter with poor performance. Performance is evaluated by means of the electromechanical coupling coefficient (K2) and the quality factor (Q) calculated from the electrical impedance frequency response plots. The results indicate that (i) K2 is insensitive to geometry (K2∼6.5%), (ii) Q increases linearly with the AlN thickness attaining Q∼1900 for a 1.7 μm thick resonator and (iii) a trampoline resonator with three beams has a better performance capability than the resonator with four or eight beams with a figure of merit K2Q∼120 and resonating at a higher frequency value than its counterparts resonators, peaking at 3.21 GHz. The performance figures agree well with those predicted by a one dimensional theory. The value of K2 also agrees well with test data but that of Q is higher than the one recorded in the lab.