Theoretical and Experimental Analysis of the Spurious Modes and Quality Factors for Dual-Mode AlN Lamb-Wave Resonators

摘要

In this paper, the spurious modes and quality-factor (Q)values of the one-port dual-mode AlN lamb-wave resonators at 500-1000MHz were studied by theoretical analysis and experimental verification.Through finite element analysis, we found that optimizing the width of thelateral reflection boundary at both ends of the resonator to reach the quarterwavelength (λ/4), which can improve its spectral purity and shift its resonantfrequency. The designed resonators were micro-fabricated by usinglithography processes on a 6-inch wafer. The measured results show thatthe spurious mode can be converted and dissipated, splitting into severallongitudinal modes by optimizing the width of the lateral reflection boundary,which are consistent well with the theoretical analysis. Similarly, optimizingthe interdigital transducer (IDT) width and number of IDT fingerscan also suppress the resonator’s spurious modes. In addition, it is foundthat there is no significant difference in the Q_s value for the two modesof the dual-mode resonator with the narrow anchor and full anchor. Theacoustic wave leaked from the anchor into the substrate produces a smalldisplacement, and the energy is limited in the resonator. Compared to theresonator with Au IDTs, the resonator with Al IDTs can achieve a higher Qvalue due to its lower thermo-elastic damping loss. The measured resultsshow the optimized dual-mode lamb-wave resonator can obtain Q_s value of2946.3 and 2881.4 at 730.6 MHz and 859.5 MHz, Q_p values of 632.5 and1407.6, effective electromechanical coupling coefficient (k_(eff)~2) of 0.73and 0.11 respectively, and has excellent spectral purity simultaneously.