Performance characterization of thin AIN films deposited on mo electrode for thin-film bulk acoustic-wave resonators

In this paper, we discuss the implementation and testing of a film bulk acoustic-wave resonator (FBAR) comprising an AlN piezoelectric thin film sandwiched between two metal electrodes and located on a silicon substrate with a low-stress silicon nitride (Si₃N₄) support membrane. In the present study, we choose molybdenum as the bottom electrode rather than the conventionally used Pt/Ti, Au/Cr or Al because it has a low acoustic attenuation and a good electrical conductivity, and provides a good adhesion between the thin AlN film and the low-stress Si₃N₄ membrane. We investigate the influence of AlN film thickness and top-electrode thickness on the resonance frequency of the FBAR device. The results indicate that decreasing the thickness of either the AlN film or the top electrode increases the resonance frequency. This suggests the potential of tuning the performance of the FBAR device by the carefully controlling AlN film thickness. Furthermore, it is also determined that the resonance frequency of the device can be increased by specifying a higher RF power for the magnetron sputtering process used to deposit the AlN film. This study employs an X-ray diffraction (XRD) technique to investigate the influence of AlN film thickness and RF power on the c-axis orientation of the thin film. It is shown that increasing either the thickness of the AlN film or the power of the RF sputtering process improves c-axis orientation. However, scanning electron microscopy (SEM) and atomic force microscopy (AFM) results reveal that a thicker AlN film exhibits a rougher surface and a larger grain size, both of which decrease the coupling factor of the FBAR device.