On the effectiveness of lateral excitation of shear modes in AlN layered resonators

We describe the fabrication and frequency characterization of different structures intended for the lateral excitation of shear modes in AlN c-axis-oriented films. AlN films are deposited on moderately doped silicon substrates covered either with partially metallic or fully insulating Bragg mirrors, and on insulating glass plates covered with insulating acoustic reflectors. TiOx seed layers are used to promote the growth of highly c-axis oriented AlN films, which is confirmed by XRD and SAW measurements. The excitation of the resonant modes is achieved through coplanar Mo electrodes of different geometries defined on top of the AlN films. All the structures analyzed display a clear longitudinal mode travelling at 11,000 m/s, whose excitation is attributed to the direction of the electric field (parallel to the c-axis) below the electrodes; this is enhanced when a conductive plane (metallic layer or Si substrate) is present under the piezoelectric layer. Conversely, only a weak shear resonance (6,350 m/s) is stimulated through the effect of coplanar electrodes, which is explained by the weakness of the electric field strength parallel to the surface between the electrodes. A significantly more effective excitation of shear modes can be achieved by normal excitation of AlN films with tilted c-axis.