Superior dual mode resonances for 1/4 λ solidly mounted resonators

The concept of the solidly mounted resonator (SMR) structure was introduced in 1965. A SMR consists of a multi-layered structure and requires material interfaces that confine waves to resonate as standing waves. Thin piezoelectric films such as AlN and ZnO with tilted texture have the capability to excite the dual mode resonance, namely, the longitudinal and shear mode resonance. To grow the tilted AlN, the substrate is placed at a variable distance from the substrate holder center in a reactive magnetron sputtering system. In addition, we tilt the off-center substrates toward the sputtering source in order to reduce the acoustic energy loss of the longitudinal wave and preserve the shear mode resonance at the same time. In this study, the 1/4 lambda mode SMR devices made with a seven-layer Mo/SiO₂ Bragg reflector and the c-axis tilted AlN are carried out. The Bragg reflector is optimized deposited with 2.28 nm RMS surface roughness, and the AlN is sputtered in appropriate sputtering pressure and appropriate substrate temperature to promote the growth of both the highly c-axis orientated and tilted AlN. The off-center deposition method evolves in a competitive growth bringing about an AlN growth pivoted in the ion-flux direction. The outcome frequency responses show dual resonant characteristics around 1.4 GHz and 2.5 GHz resulted from the shear and longitudinal resonances, respectively. We successfully improve the longitudinal resonance by tilting the substrate toward the sputtering source. Not only the shear resonance for the liquid media sensing application, but also an outstanding longitudinal resonance could be obtained. The superior dual mode resonances are realized.