On the lateral excitation of shear modes in AlN layered resonators

Abstract

Abstract-In this paper we describe the fabrication and frequency characterization of different structures intended for the lateral excitation of shear modes in A1N c-axis-oriented films, which are at the same time designed to minimize the excitation of longitudinal modes. Laterally excited resonators were built on partially metallic (Si0 2 , W) and insulating (SiOC, Si 3 N 4 ) acoustic mirrors built on silicon substrates, and on insulating mirrors (Si0 2 , TaO x) built on insulating glass plates. TiO x seed layers were used to stimulate the growth of highly c-axis oriented A1N films, which was confirmed by XRD and SAW measurements. Coplanar Mo electrodes of different geometries were defined on top of the A1N films to excite the shear modes. All the structures analyzed displayed a clear longitudinal mode, corresponding to an acoustic velocity of 11000 m/s, but a null or extremely weak shear response corresponding to a sound velocity of around 6350 m/s. The simulation of the frequency response based on Mason's model confirms that the shear resonance is extremely weak. The observed longitudinal modes are attributed either to the field applied between the electrodes and a conductive plane (metallic layer or Si substrate) or to the electric field parallel to the c-axis in the edges of the electrodes or in tilted grains. The low excitation of shear modes is attributed to the very low values of electric field strength parallel to the surface

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

“…The most straightforward, known as lateral 59 excitation, consists in applying an electric field parallel to the sur-60 face of a c axis oriented through two coplanar electrodes, to excite 61 the shear wave by virtue of the d 15 piezoelectric coefficient [5]. In 62 practice, lateral excitation provides extremely weak electric fields 63 between the electrodes and, hence, hardly noticeable shear excita-64 tions [6,7]. Alternatively, shear modes are effectively generated by the direction of the impinging species [8].…”