Growth of AlN thin film on Mo electrode for FBAR application

In this study, a FBAR device with four-layered composite structure is studied. In this four layer structure, an AlN piezoelectric thin film is sandwiched between two metal electrodes, all of which lied on a low stress silicon nitride (Si₃N₄) as a support membrane in silicon. Most of the researches are using Pt/Ti and Al as the bottom electrode. But in this study, molybdenum was chosen as the bottom electrode because it has low acoustic attenuation, good electrical conductivity and has a good adhesive with AlN and low stress silicon nitride. Compare to Pt/Ti bi-layer electrode, the process of Mo electrode is easier. Compare to Al, there is no evidence transition region between Mo and AlN film which is indicative of smooth interface quality to fabrication FBAR. The high quality c-axis orientation AlN thin films with different thickness are achieved by optimum sputter deposition conditions on Mo and showed quasi-single crystal piezoelectric properties assessed using XRD, SEM, AFM and electrical characterizations are reported. The experimental results indicate that the resonance frequency is mainly determined by the thickness of the AlN film layer, which reducing the AlN thickness increase the resonant frequencies. One of the devices considered here with AIN film thickness is 1.35μm, Mo and Al is used as bottom electrode and top electrode with the thickness of 0.1μm and 0.18μm, respectively. The resonant frequency of the devices is 3.42 GHz and the return loss is -37.95 dB.