Analysis of spurious peaks at series resonance in solidly mounted resonators by combined BVD-Mason modelling.

Solidly Mounted Resonators (SMRs) for high frequency RF filters and sensing applications often display spurious resonances that distort their frequency response. In this work, we try to identify the origin of spurious resonances accompanying the main series resonances in AlN-based SMRs with the help of modified Butterworth Van Dyke (BVD) and Mason's models. By manufacturing SMRs of different sizes and shapes and studying the influence of the position of the electrical probing spot, we have demonstrated both theoretically and experimentally that devices with larger areas are more likely to display these additional peaks. Our updated models accurately simulate the frequency response of the SMRs, revealing that spurious peaks are mostly related to the resistance of the electrodes. Our study clarifies the origin of the spurious resonances and offers solutions for both, the optimal design and measurement method of SMRs.

Assessment of the shear acoustic velocities in the different materials composing a high frequency solidly mounted resonator.

Thin film acoustic resonators operating in the shear mode are being increasingly used for in-liquid sensing applications. A good design of such sensors requires accurate knowledge of the acoustic properties of the materials composing the whole device, which specifically includes their shear velocities. Here we present a method to assess the shear acoustic velocity of high and low acoustic impedance films commonly used in AlN-based solidly mounted resonators (SMRs), using test devices specifically designed to induce a half-wavelength resonance in the layer under study. Provided that the thickness and mass densities of all the layers are known, fitting the electrical response by Mason's model over a wide frequency range gives accurate values of both longitudinal and shear mode velocities. The assessment of porous and dense SiO2, Mo, W and Ta2O5 sputtered films yields shear velocities of 3150m/s, 3950m/s, 3450m/s, 3350m/s and 2900m/s, respectively. In addition, the resonances stimulated in the Ir and Au top electrodes enable deriving their shear modes velocities, with values of 3950m/s and 2350m/s, respectively.

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.