Aluminum Nitride Cross-Sectional Lamé Mode Resonators

This paper demonstrates a new class of AlN-based piezoelectric resonators for operation in the microwave frequency range. These novel devices are identified as cross-sectional-Lamé-mode resonators (CLMRs) as they rely on the piezoelectric transduction of a Lamé mode, in the cross section of an AlN plate. Such a 2-D mechanical mode of vibration, characterized by motion along both the lateral and the thickness directions, is actuated and sensed piezoelectrically through the coherent combination of the e
₃₁
and e
₃₃
piezoelectric coefficients of AlN. This special feature enables the implementation of CLMRs with high values of electromechanical coupling coefficient. In particular, we experimentally demonstrated k
_t
²
values in excess of 4.6% and 2.5% in CLMRs using, respectively, two or one metallic interdigitated metallic electrodes, and operating around 1 and 2.8 GHz. Furthermore, despite the dependence of the cross-sectional Lamé mode on both the thickness and the width of the AlN plate, lithographic tunability of the resonance frequency, by changing only the in-plane dimensions of the device, can be achieved without a substantial degradation of k
_t
²
. The capability of achieving high k
_t
²
and multiple operating frequencies on the same chip, without additional fabrication costs (lithographic tunability of the resonance frequency), makes this technology one of the best candidates for the implementation of multifrequency and low insertion loss filter banks for reconfigurable radiofrequency front ends.