Aluminum nitride lamb wave resonators with high figure of merit for narrowband filter applications

In this paper, we report experimental results on band-pass aluminum nitride (AlN) MEMS filters that are based on Lamb wave resonators (LWRs) which exhibit a high ratio of the resistance at parallel resonance (R_p) to the resistance at series resonance (R_s). One-port LWRs working at S0 mode with various geometry dimensions are analyzed and the design guidelines for LWRs with high performance are reported for the first time. A 252 MHz LWR, with an aperture of 200 μm, 12 IDT fingers, and 1.5 μm thick AlN, is found to have a remarkable R_p/R_s of 1317 and a corresponding product of the electromechanical coupling coefficient (Kf) and quality factor (Q) exceeding 60. Consisting of such resonators, a 6-stage ladder filter with a low pass-band insertion loss (IL) of 4.5 dB, a high out-of-band rejection of 40 dB, and steep filter skirts, is achieved, occupying only a small area of 880×1150 μm².